Ralts is imho the best Psychic-type to use in the main storyline. Alakazam is unobtainable for many people, Bronzor sucks, etc. Top-tier.
Ralts is obtained on either Route 208 or Route 209 (around Hearthome city). It evolves at level 20 into Kirlia, so only a couple of levels after you catch it. It then evolves into Gardevoir at level 30 (pretty early for such a fantastic Pokemon) or Gallade if you use the Dawn Stone (available a bit later on). Ralts is fairly common, you have a 20% chance to encounter it on Route 209. One of the best things about Ralts is that you can choose the evolution depending on its nature. If you have an Atk increasing nature or a SpA decreasing one and it's male, it's probably best to evolve it into Gallade (which gets Psycho Cut at level 31, Leaf Blade/Night Slash with a Heart Scale, and Close Combat at level 53. If you want a Fighting-type move earlier on, you can always teach it Drain Punch or Brick Break.
I however prefer Gardevoir. She can steamroll practically any boss in the game by herself, and is extremely self-sufficient. She learns her excellent moves very early on (Calm Mind as a Kirlia, Psychic at level 33, and Wish with a Heart Scale). A set of Calm Mind, Psychic, Thunderbolt (she's a great Pokemon to use the TM on) and Wish/Shadow Ball/Focus Blast can really massacre any boss. She's obviously no slouch when it comes to one on one either, as that 125 SpA makes quick work of anything with Psychic or a coverage move. She's awesome at taking special hits as well. Trace is an incredible ability, even in-game, and it makes her one of the best Gyarados counters (these guys are notoriously difficult in-game), as she bounces back Intimidate and strikes them down with Thunder. Synchronize is fine too, but if you can, catch a Ralts with Trace.
Gallade and Gardevoir only have one real flaw, and that's their low Exp gain. At 1,250,000, their EXP Growth is slow, but the fact they get their good moves and evolutions early on means that this really isn't much of an issue. Exp share is a great way to bring Ralts up as well.
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