it's the new servant, sitonai, who is a pseudo-servant bound to illya. she showed up in the last lostbelt story chapter, but wasn't summon-able until now. sitonai is a figure from ainu mythology, hence the clothing. she also apparently has elements of louji and freyja from scandinavian mythology, but that just sounds like something that was in the lostbelt chapter, since nothing in her playable form references that as far as i can tell.

as far as being culturally insensitive, idk. i haven't seen anyone more knowledgeable on ainu culture speaking out against it. with pseudo-servants, you're not really getting a representation of the original myth most of the time anyways, just the aesthetic trappings of them (see also; ishtar, parvati, muramasa, zhuge liang, etc).

personal opinion, i don't think her design is any worse than the ainu sisters from the samurai shodown games (nakoruru and rimururu) and those were historically positively received by the ainu, as far as i've read about it.