I realized that this thread did not yet have a trainer look-up cheatsheet, like there used to be in the past. Let's fix that!
Following traditions I try to summarize the roster of every trainer that appears past battle 40. As an added bit of information, I've marked duplicate rosters between trainers by underlining the first character of the trainer class and putting duplicate trainers on adjacent lines - if the descriptions for two adjacent trainers are the same, but not underlined then their rosters are not identical. This is not a replacement for looking up full rosters, but rather a cheatsheet for quick checks and learning overall trainer patterns.
Note that I didn't double check everything, if there's anything wrong or additional brief words could better describe a particular trainer's roster I'm all ears. Also I broke the whole post pasting it in and out of my text editor lol. Arbitrary spacing intentional, for grouping together "related" trainers within a trainer class and separating archetypes within them.
Edit: new details and corrections added with input from Smuckem, paperquagsire and ReptoAbysmal, much appreciated! Also a new edit, the relations and (lack of) overlap between the various speedmon trainers were figured out. April 10 2018: Possibly the last edit for now, adding descriptions for a few more trainers and fixing minor errors. September 2018: a set of corrections and additions. December 2018: New details thanks to HeadsILoseTailsYouWin. January 2019: New details & trainer list reordered and categorized by general trainer types: type specialists, Weather, legendaries, neutral, etc
True Neutral: trainers with no bias towards a specific type or theme, note that all of these are primarily Set4 only.
These seven have some things in common: none of them run a single Trick Room setter. Most non-legendary species in Tree are among these seven's rosters, but the Pokémon on the Scientist Cal/Robyn roster are absent. Further investigation is being done on what exactly defines these rosters, which are the largest and most varied in the Tree.
Police Officer
Rendor - Set4 - large roster, Set34 Electivire + Rotom-Wash
Dancer
Carrie - Set4 - large roster
Black Belt
Chucky - Set4 - large roster, Set34 Lickilicky + Magmortar + Rotom-Fan + Probopass
Aether Foundation
Haley - Set4 - large roster, Set34 Magmortar + Rotom-Wash + Probopass
Aether Foundation
Heidi - Set4 - large roster, Set34 Lickilicky + Rotom-Wash + Electivire
Aether Foundation
Harvey - Set4 - large roster, Set34 Lickilicky + Rotom-Frost + Electivire
Aether Foundation
Luke - Set4 - large roster, Set34 Magmortar + Rotom-Frost + Probopass
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Type Specialists: various levels of preference into particular types of Pokémon
Ace Trainer
Bette - BST500+ Set34 mons, Fighting/Poison/Ghost preference + Rotom-Wash, Magmortar - large roster
Ace Trainer
Poppy - BST500+ Set34 mons, Flying/Dark/Steel preference + Kommo-12, Rotom-Fan - large roster
Ace Trainer
Tamah - BST500+ Set34 mons, Normal/Ground/Fairy preference + Rotom-Fan - large roster
Ace Trainer
Hashim - BST500+ Set34 mons, Water/Bug/Rock preference + Kommo-o12, Rotom-Fan - large roster
Ace Trainer
Levi - BST500+ Set34 mons, Fire/Psychic preference + Mamoswine, Rotom-Wash, Starmie, Walrein, Exeggutor (both Formes)
Ace Trainer
Munin - BST500+ Set34 mons, Electric/Grass/Dragon preference - large roster
Bellhop
Donna - Set34, soft Fighting & physical attacker preference with plenty of other types and some special attackers - large roster
Bellhop
Gilroy - Set34, soft Fighting & physical attacker preference with plenty of other types and some special attackers - large roster
Cook
Larry - Set34 mons, soft Fire/Water/Grass/Ice preference - large roster
Cook
Tony - Set34 mons, soft Fire/Water/Grass/Ice preference - large roster
Firefighter
Calder - Set34, soft Water-type preference with slight amounts of other types - large roster
Firefighter
Presta - Set34, soft Water-type preference with slight amounts of other types - large roster
Janitor
Paulo - Set34 Poison + Water/Rock/Ghost/Ground soft preference, Janitor themed - large roster
Janitor
Sika - Set34 Poison + Water/Rock/Ghost/Ground soft preference, Janitor themed - large roster
Office Worker
Darrel - Set34, soft Fairy/Normal preference with slight amounts of other types, Set3 Pidgeot & Toucannon - large roster, no overlap with the Punk Girl roster (opposite archetypes)
Office Worker
Jana - Set34, soft Fairy/Normal preference with slight amounts of other types, Set3 Pidgeot & Toucannon - large roster, no overlap with the Punk Girl roster (opposite archetypes)
Punk Girl
Edda - Set34, soft Poison/Dark preference with slight amounts of other types - large roster, no overlap with the Office Worker
roster (opposite archetypes)
Punk Girl
Zed - Set34, soft Poison/Dark preference with slight amounts of other types - large roster, no overlap with the Office Worker
roster (opposite archetypes)
Office Worker
Harding - Set34, Steel/Electric/Psychic preference + Druddigon3, Abomasnow3, Porygon2&Z, Shuckle - large roster - same as Savir minus Mawile
Office Worker
Savir - Set34, Steel/Electric/Psychic preference + Druddigon3, Abomasnow3, Porygon2&Z, Shuckle - large roster - same as Harding plus Mawile
Worker
Dan - Set34, Rock/Ground/Steel/Electric exclusively - large roster
Worker
Omar - Set34, Rock/Ground/Steel/Electric exclusively - large roster
Pokémon Center Lady
Perri - Set234, Normal/Fairy/Psychics that are Pink or White + Togedemaru, Lilligant, focused roster
The Terrain Quartet: 4 veterans which specialize in a particular Terrain move, and by extension its associated type and have legendaries of that type
Veteran
Kikujiro - Set34, Electric exclusively + Set12 Electric Legendaries, Togedemaru2 (only set2!) - small roster
Veteran
Aino - Set34, Psychic preference + Rotom-Wash, Uxie12, Oranguru2 (only Set2!) - small roster
Veteran
Xio - Set34, Fairy exclusively + Florges234 + Sylveon134 - small roster
Veteran
Xenophon - Set34, Grass preference + Virizion12, Comfey3, Florges2
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Themed Trainers: Eeveelutions, Starters, Specific Stats, "trainer archetypes that aren't Type Specialty"
Golfer
Bruce - Set34 Eeveelutions, excluding Sylveon
Golfer
Moe - Set34 Eeveelutions, excluding Sylveon
Preschooler
Reina - Set34 Starters (all 7 generations)
Preschooler
Niara - Set34 Starters (all 7 generations)
Punk Guy
Dustin - Set34, Intimidate users + Rotom-Fan34 - small roster
Punk Guy
Etta - Set34, Intimidate users + Rotom-Fan34 - small roster
Collector
Dennis - Set34, Collector-themed species: think in-game Collector trainers, fossils, horned Pokémon, Safari Zone regulars, pseudo-legendaries, one-off/hard to catch Pokémon - large roster
Collector
Sam - Set34, Collector-themed species: think in-game Collector trainers, fossils, horned Pokémon, Safari Zone regulars, pseudo-legendaries, one-off/hard to catch Pokémon - large roster
Sightseer
Alice - Set34, 140+ Base Atk Pokémon + Regigigas1234
Sightseer
Chen - Set34, 140+ Base Atk Pokémon + Regigigas1234
Sightseer
Christian - Set34, 130+ Base SpA Pokémon + Latios1234, Heatran1234
Sightseer
Ezra - Set34, 130+ Base SpA Pokémon + Latios1234, Heatran1234
Golfer
Susanna - Set1234 Zapdos, Regigigas & Entei + Set34 100 Base Speed mons, Ninetales-Alola (no Comfey)
Golfer
Patrick - Set1234 Zapdos, Regigigas & Entei + Set34 100 Base Speed mons, Ninetales-Alola + Comfey
Youth Athlete
Buddy - Set34, 122+ Base Speed mons - small roster
Youth Athlete
Leena - Set34, 122+ Base Speed mons - small roster
Ace Trainer
Granville - Set34, 120+ Base Speed mons - a superset of Buddy/Leena with Sceptile, Alakazam, Rotom-Heat4, Dugtrio2 (both formes) added
Ace Trainer
Raz - Set34, 120+ Base Speed mons - a superset of Buddy/Leena with Sceptile, Alakazam, Rotom-Heat4, Dugtrio2 (both formes) added
Youth Athlete
Hilario - Set34, 112-122 Base Speed mons + Raikou1234 - overlaps Sceptile/Alakazam/Greninja with Granville/Raz, includes slower (down to base 112) Pokémon
Youth Athlete
Thamina - Set34, 112-122 Base Speed mons + Raikou1234 - overlaps Sceptile/Alakazam/Greninja with Granville/Raz, includes slower (down to base 112) Pokémon
---
Weather trainers - 4 for each of Sand/Sun/Rain, two soft Hail users
Hiker
Stellan - Steel/Rock/Ground preference, Gigalith234 + Tyranitar234, Terrakion1234 + Regirock1234 + Cresselia4, TR setters running only the set with TR - large roster - Garchomp4, Excadrill3, Steelix4 Landorus3 notable Sand-boosted threats
Hiker
Vivek - Steel/Rock/Ground preference, Gigalith234 + Tyranitar234, Terrakion1234 + Regirock1234 + Cresselia4, TR setters running only the set with TR - large roster - Garchomp4, Excadrill3, Steelix4, Landorus3 notable Sand-boosted threats
Police Officer
Benji - Sand specialist, all sets of Sand Stream mons, rest either 1 or 2 possible sets, focused roster - Garchomp34, Excadrill34, Steelix4 notable Sand-boosted threats
Police Officer
Lou - Sand specialist, all sets of Sand Stream mons, rest either 1 or 2 possible sets, focused roster - Garchomp34, Excadrill34, Steelix4 notable Sand-boosted threats
Dancer
Atalanta - Rain specialist, focused roster with Swift Swim/Rain Dance/Drizzle, all sets of Pelipper and Politoed
Dancer
Variel - Rain specialist, focused roster with Swift Swim/Rain Dance/Drizzle, all sets of Pelipper and Politoed
Rising Star
Joaquin - Rain specialist, Set34 Swift Swim users, Set234 Politoed + Pelipper, Suicune1234, Tornadus4, Zapdos3, Magmortar34 - of Pokémon with Rain Dance, only the set with Rain Dance is included
Rising Star
Dolly - Rain specialist, Set34 Swift Swim users, Set234 Politoed + Pelipper, Suicune1234, Tornadus4, Zapdos3, Magmortar34 - of Pokémon with Rain Dance, only the set with Rain Dance is included
Ace Trainer
Lea - Set34, Sun specialist with large roster, Chlorophyll users, Ninetales12, Charizard34, Houndoom4, Arcanine3, Lurantis4, Darmanitan4, Typhlosion4
Ace Trainer
Sylvia - Set34, Sun specialist with large roster, Chlorophyll users, Ninetales12, Charizard34, Houndoom4, Arcanine3, Lurantis4, Darmanitan4, Typhlosion4
Rising Star
Marianne - Sun specialist with focused roster, Set34 Chlorophyll users, Ninetales12, Charizard3, Moltres1234, Heatran123, Houndoom4, Arcanine3, Lurantis4, Delphox3, Darmanitan34, Rotom-Heat34
Rising Star
Erix - Sun specialist with focused roster, Set34 Chlorophyll users, Ninetales12, Charizard3, Moltres1234, Heatran123, Houndoom4, Arcanine3, Lurantis4, Delphox3, Darmanitan34, Rotom-Heat34
Black Belt
Arnold - Set4 - large roster, Set24 Snow Cloak & Snow Warning species + Rotom-Fan34, Beartic2
Black Belt
Iniko - Set4 - large roster, Set24 Snow Cloak & Snow Warning species + Rotom-Fan34, Beartic2
---
Slow Pokémon: exclusively species with low base Speed, often with Trick Room setters
Scientist
Robyn - Set34 + Set4 Slow Pokémon, around 50/50 Set34/Set4 split across species
Scientist
Tivon - Set34 + Set4 Slow Pokémon, around 50/50 Set34/Set4 split across species
Scientist
Cal - Set34 + Set4 Slow Pokémon, around 50/50 Set34/Set4 split across his species - similar to Robyn and Tivon, but missing Lickilicky
Scientist
Stein - Set34 + Set4 Slow Pokémon, around 50/50 Set34/Set4 split across species, TR setter species ONLY run the set with TR excluding Bronzong34, rest of them Set34
Scientist
Cadel - Set34 + Set4 Slow Pokémon, around 50/50 Set34/Set4 split across species, TR setter species ONLY run the set with TR excluding Bronzong34, rest of them Set34
Pokémon Breeder
Izel - Set34, Slow Pokémon, incl. Drampa - only 1 TR setter, Bronzong34
Pokémon Breeder
Danby - Set34, Slow Pokémon, incl. Drampa - only 1 TR setter, Bronzong34
Pokémon Breeder
Lori - Set34, Slow Pokémon, incl. Amoonguss and Shiinotic, multiple TR setters
Pokémon Breeder
Colby - Set34, Slow Pokémon, incl. Amoonguss and Shiinotic, multiple TR setters
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Legendary Pokémon trainers: rosters with many Legendary Pokémon, and sometimes pseudo-legendaries and other high-BST species. Split by which sets of legendaries they run.
Set12 Legendaries
Veteran
Priya - Set12 Legendaries - small roster - both Cresselia12 and Zapdos12, the most common trainer to bring either
Veteran
Placido - Set12 Legendaries - small roster
Lass
Inez - Set12 Legendaries - large roster
Backpacker
Gwenny - Set12 Legendaries, includes Cresselia12 - small roster - same roster as Youngster Brady
Youngster
Brady - Set12 Legendaries, includes Cresselia2 - small roster - same roster as Backpacker Gwenny
Youngster
Napoleon - Set12 Legendaries, includes Zapdos2 - large roster
Black Belt
Boris - Set34 BST530+ Black Belt pool + Set12 Musketeers, Regigigas, Heatran - large roster
Black Belt
Bryson - Set34 BST530+ Black Belt pool + Set12 Genies & Regi Trio - large roster
Dancer
Jo - Set34 BST530+ Dancer Pool + Set12 Kanto birds, Lati@s, Uxie - large roster
Dancer
Tasanee - Set34 BST530+ Dancer Pool + Set12 Johto dogs, Cresselia, Mesprit - large roster
Boris, Bryson, Jo, and Tasanee have a unique connection - they run regular species with BST equal or greater than 530, divided into two sets, and the two pools though similarly BST-themed have no overlap between them. The full lists of Pokémon this encompasses are as follows:
BST530+ Black Belt Pool = Aggron, Blastoise, Blaziken, Charizard, Chesnaught, Delphox, Empoleon, Feraligatr, Florges, Gogoat, Greninja, Haxorus, Hydreigon, Infernape, Kingdra, Magmortar, Mamoswine, Noivern, Rhyperior, Rotom (Heat Rotom), Sceptile, Slaking, Swampert, Tangrowth, Togekiss, Typhlosion, Tyranitar, Volcarona, Walrein
BST530+ Dancer pool = Arcanine, Archeops, Blissey, Crobat, Decidueye, Dragonite, Exeggutor (Alola Form), Exeggutor, Garchomp, Golisopod, Goodra, Gyarados, Incineroar, Lapras, Magnezone, Metagross, Milotic, Porygon-Z, Primarina, Rotom (Mow Rotom), Salamence, Snorlax, Vanilluxe, Wishiwashi (School Form)
Set34 Legendaries
Backpacker
Fernanda - Set34 Legendaries - small roster
Veteran
Candy - Set34 Legendaries + Kommo-o34 - small roster
Veteran
Ignacio - Set34 Legendaries + Kommo-o34 - small roster
Veteran
Demiathena - Set34 Legendaries - small roster
Lass
Samantha - Set34 Legendaries - smaller roster
Veteran
Dooley - Set34 Legendaries + Set34 pseudo-legendaries, strong Megas, and the odd Sylveon - large roster
Preschooler
Naya - Set34 Pseudo-Legendaries + Set34 Legendaries + Kommo-o1234
Preschooler
Victor - Set34 Pseudo-Legendaries + Set34 Legendaries + Kommo-o1234
Set1234 Legendaries
Gentleman
Mechabob - Set1234 Musketeers, Genies, Johto Dogs & Regis incl. Gigas + Electivire
Gentleman
Abel - Set1234 Musketeers, Genies, Johto Dogs & Regis incl. Gigas + Electivire
Madame
Donny - Set1234 Kanto birds, Lake trio, Cresselia, Heatran & Latis + Kommo-o
Madame
Gracie - Set1234 Kanto birds, Lake trio, Cresselia, Heatran & Latis + Kommo-o
Ace Trainer
Horace - Set1234 Regi Trio (NOT Regigigas) & Lake Trio + Kommo-o1234 - small roster
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Special Trainers: the story characters who appear every 10 battles
Pokémon Trainer
Anabel - Set34 Lucario, Snorlax, Alakazam + Set1234 Entei, Latios, Raikou
Pokémon Trainer
Cynthia - Set34 Garchomp, Togekiss, Lucario, Milotic, Spiritomb
Pokémon Trainer
Grimsley - Set34 Absol, Bisharp, Drapion, Houndoom, Liepard, Scrafty, Sharpedo, Tyranitar + Honchkrow3 (Set3 only)
Pokémon Trainer
Wally - Set34 Altaria, Gallade, Garchomp, Magnezone
(Only in USUM)
Pokémon Trainer
Kukui - Set34 Decidueye, Incineroar, Primarina, Braviary, Magnezone, Snorlax + Ninetales-Alola12, both Lycanroc2-Midday and Midnight
(Only in Sun/US)
Pokémon Trainer
Kiawe - Set34 Arcanine, Kangaskhan, Salazzle, Talonflame + Marowak-Alola2
Pokémon Trainer
Plumeria - Set34 Crobat, Gengar, Lurantis, Salazzle, Toxapex
Pokémon Trainer
Sina - Set34 Abomasnow, Drampa, Glaceon, Lilligant, Mandibuzz, Oranguru, Sandslash-Alola2
(Only in Moon/UM)
Pokémon Trainer
Dexio - Set34 Braviary, Espeon, Passimian, Slowbro, Turtonator, Whimsicott + Ninetales-Alola2
Pokémon Trainer
Guzma - Set34 Bisharp, Golisopod, Honchkrow, Liepard, Pinsir, Toxapex, Toxicroak
Pokémon Trainer
Mallow - Set34 Sceptile, Toucannon, Talonflame, Tsareena, Trevenant, Comfey, Lurantis