Level-up movepool:
Having two start moves is reasonable and typical if Necturna were to be the kind of Pokemon catchable in the early routes of the game. Interestingly, though, if Necturna were to be catchable only later in the game, (or hatched from an Egg made by Ditto-breeding by a mother with 4 moves,) then Necturna would not receive any Physical Grass STAB until level 49. Necturna is intended to be weak and "sweet" early on, but become more malevolent later, like a jilted girlfriend might be. Necturna learns Future Sight at Level 15, at which point she learns of her fate to be alone, presumably she thinks because her companion will leave her. She then learns Follow Me at Level 19 in order to lure her companion away from the public. Every move learned after that is used to preemptively attack her companion before she can be rejected. Little does she know that the Future she envisioned would not have happened if she had not made it come true. Perish Song, learned at Level 75, signifies the death of the last shreds of goodness left in Necturna's heart. Finally, at level 100, Necturna has gained all the experience she can, and she finally understands the Gravity of her situation. But it is too late for her to fix the past. As for total count, mine stands at 18 moves which was the very max I was willing to include in Level-up. That is quite large, tbh, and is probably too large, but this is what fits the flavor best.
TMs/HMs:
The most important thing for me to do here was to establish consistency with the game. I am a stickler for Flavor fitting what GF would actually be likely to do. Therefore, Toxic, Hidden Power, Protect, Frustration, Return, Attract, Solarbeam, Shadow Ball, Shadow Claw, Facade, Rest, Double Team, Hyper Beam, Giga Impact, Substitute, and Round are all musts for inclusion in my books. And I will have a very hard time voting for any movepool that does not include all of these moves. Yes we are operating with a cap on total number of moves, but these all should make every list, even above extraneous personal flavor inclusions if necessary. Beyond those, Calm Mind, Energy Ball, Will-O-Wisp, Grass Knot, and Strength all feel like fairly obvious inclusions, that I think all movepools should contain, but I could look past if they were not. The only personal flavor TMs/HMs I've included are Swagger and Torment. I think that Necturna's design has lots of swag, and I can't imagine someone reading over my description of the Level-up movepool and disagreeing with my inclusion of Torment.
Egg Moves:
I placed Necturna in the Grass Egg Group, where I think she would actually be placed by GF. On a pure flavor level, Necturna belongs in that Egg Group, where she breeds with her OBVIOUS relative Carnivine (a la Luvdisc and Alomomola or Tauros and Bouffalant.) I could have placed Necturna in both the Grass Group and the Field Group, in order to get Sketch by Egg Move and avoid all illegalities, but that didn't feel right to me as the Field Group is mostly mammalian and there is not a single Ghost-type in the entire group. True, this caused some difficulty in Egg Move combinations, but it also accomplished more than making things tricky. It actually forced me to learn about breeding and egg groups, a subject I found confusing. Through the Egg Moves, I have complemented Necturna's movepool with a lot of flavor (Bite, Captivate, Dream Eater, Spite,) and some potentially competitive moves (Stone Edge, Toxic Spikes, Switcheroo, Gravity, Grasswhistle, Extrasensory, Curse.) Despite the difficulties of breeding outside of the Field group, Necturna still only has two combinations that appear legal at first sight that are in fact illegal, neither of which is competitively viable anyway. EDIT: Toxic Spikes has been added to the Egg Movepool, since that is the most logical method for Necturna to get T-Spikes. Stone Edge was made an Egg Move too, since no Female-only Pokemon has ever been able to learn Stone Edge, so breeding seemed to be the least intrusive method to give Necturna Stone Edge.
General Flavor:
We are working with a CAP named Necturna. Based on the word Nocturnal. Therefore I found it important to include any night-based move I could find (with the exception of Night Daze, a Zoroark Exclusive.) That amounted to Night Shade, Night Slash, and Moonlight. But one piece of information that people may not know is that it could also be based on a Nocturne, which is "a short composition of a romantic or dreamy character suggestive of night, typically for piano," or, in short, music. Therefore aside from the required Round, I tried to include several other sound-based moves. Screech, Grasswhistle, and especially Perish Song fit that flavor perfectly. Additional flavor considerations went to moves that fit a female-only Pokemon well, namely Attract, Captivate, and Sweet Scent. (and to a lesser degree Spite, don't hate me female Smogonites!) Strength was included as having a Pokemon with 120 Base Attack that doesn't learn Strength seemed odd to me. The abilities Forewarn and Telepathy dictated Calm Mind, Captivate, Dream Eater, Extrasensory and Future Sight. Finally, the remainder of the flavor was based on the physical depiction of Necturna in the design. The teeth-stitch things lent themselves to Bite and Super Fang, the fake eyes to Leer and Scary Face, etc.
SKETCH SCENARIO:
As I said above, at night, go to the First Floor of the Relic Castle with a Necturna with Night Shade in your party.
The painter is facing the wall with only one blank space in between. His focus is steely. You cannot try to talk to him. You must deliberately walk between him and the wall carvings he is painting. If you do not have a Necturna with Night Shade in your party, he will simply yell at you, "Watch where you're walking! I'm painting here!" If you do have a Necturna with Night Shade in your party, he will instead ask you, "Are you interested in what I'm painting?" You will be prompted to answer Yes or No. If you answer No, he will then tell you, "Then kindly leave me alone to my painting." If you answer Yes, he will happily say, "Good. Very few youngsters are interested in art. Do you know how to paint?" You will be prompted to answer Yes or No. If you answer Yes, he will say, "Oh. Well then I guess there's nothing for me to teach you." If you answer No, he will ask you, "Would you like to learn?" You will be prompted to answer Yes or No. If you answer No, he will disgustedly ask you, "How can you be interested in art but not want to learn?! Kids! grumble..." If you answer Yes, he will say, "Great! Here's the trick. Have your Pokemon sketch everything for you. Then all you have to do is color it in. I actually don't have any artistic talent! My Smeargle is the real artist. Do you have any Pokemon that can learn to Sketch?" Displays your Pokemon with "Not Able" for all other Pokemon and "Able" for Smeargle (if you have one) and Necturna, which you have to have. If you select Smeargle, he will say, "But you've been lying! Smeargle already knows how to Sketch." (even if it doesn't have Sketch itself at that moment) If you select Necturna, he will say, "Perfect, your Necturna looks like she has a lot of natural artistic ability!" Goes to the "which move should be forgotten sequence." The painter says, "Here. Take this Ribbon too. This ribbon will show everyone just how much you love your Necturna." Attaches Classic Ribbon, which prevents trading. "If you actually want to learn how to paint, you should go talk to Burgh in Castelia city. I heard he actually paints without the help of a Pokemon Sketcher!"
If you try to come back and get the same Necturna taught Sketch, (after going through the same steps above up until the Select Pokemon screen with already-learned-sketch Necturna still listed as Able,) he will say, "But I've already taught your Necturna Sketch! I remember giving her that Ribbon!" Other Necturna will still be tutorable.