+ Quiver Dance
Stats + Typing:
It's a Bug type which give is a very likely chance to use something like Quiver Dance. Sure it's not exactly a butterfly but then you have the case of Lilligant so it's not too much of a stretch for a firefly to get it.
Really just needed something good to abuse Prankster with and since Tail Glow is taken by Volbeat...
Makes more sense to get Quiver Dance than Lilligant (firefly is close enough to a butterfly). I can imagine Illumise using Quiver Dance movements to attract Volbeat so it works in my head lol
Illumise has a very poor movepool and it can't even use Growth with Baton pass + Prankster since it's an illegal combination. At least with a Quiver Dance it would have a niche with Prankster and it's not totally overshadowed by Volbeat in the Prankster passing and at least it has the better stats of the two to actually use the boosts if it had to use it since Illumise stats are more specially based while Volbeat is more physical.
Stats + Typing:
Shiftry is offensively based and while it's Special Attack is the lower of the two attacking stats, it can still be decent.
Shiftry likes the sun to abuse Chlorophyll, but if the opponent decides to change rain for his favour at least Shiftry can still abuse the sudden weather change with Hurricane.
It has fans for hands and is repeatedly stated to create huge gusts of wind.
It's not really a huge need for it since it has a pretty good movepool anyway. Just another option for coverage/abusing weather. Would be the best move to use against Ludicolo though.