youre clearly getting Dougie's Avatar so i am not going to bother buttering him up.
Seamus' avatar, i dont recall much. but i believe he makes terrifying seem viable and, you know, smashes face which makes the belles even more valuable to him. He kind of does the same being all tanky but with some obvious differences here and there.
Nicodem, honestly, i thought he had a weird avatar. His avatar seems to, IMO, to be doing the exact same thing, but better, like, a LOT better. So if you dont like playing him, save on buying this guy for later.
Kirai is a great master to play with an.......interesting avatar. My problem with her is that 6-10ss are going to be spent on Seishin (though they are a great cannon fodder/support model). Her avatar is what it looks like: Ikiryo on steroids/growth hormones. See, this means that kirai, once manifested, has lost on some of her abilities. She loses the ability to summon (though she can make seishin in the ezxact same way still) and some movement shenanigans (though she still has some, but only some of them are reliable imo) in return she gets the ability to hurt. So you have to absolutely certain more things dont need to be summoned when manifesting. Id recomend buying her only if you really think kirai is fun to play with seeing as her avatar is more of a last resort