I think it's very important for Marisa's character that she
isn't a youkai. I've written about it over
here (ignore the narrative bit, it's not very good) but to sum up- Marisa is, all in all, a completely normal human. The others all have certain gifts (shrine maidens have their gods, Youmu is half-ghost, etc) that contribute to their abilities, but Marisa is as human as anyone in the village. It's testament to her absolute dedication and hard work that despite this, she can happily go toe-to-toe with gods, monks, shrine maidens, vampires, immortals, etc and still defeat them. She's a relentlessly hard worker, and she has
juuuust enough recklessness to take the silly risks necessary to be the best. Explaining that away by giving her youkai powers would diminish that too much, I think.
A story where she becomes a youkai could be an interesting read if done well, though. Would she give up her humanity for power?