Momiji (紅葉, "Japanese Maple") is a young priestess who is tricked by Tsubaki into fighting against Inuyasha's group. She is Botan's sister, and is about 15 years old.
Momiji and her sister are both sort of rookie priestesses, so they have a hard time trying to defeat the Inuyasha group. Neither of them give up easily, they are only defeated when an explosion sends the two flying backwards and knocks them unconscious for a while.
Momiji and her sister seem to have a great liking for cute things. They love the looks of Kirara and Shippō, and whisper for a long time over whether to remove the movement restraint they put on Inuyasha because it knocked Shippō and Kirara over, too.
(Source: Inuyasha Wikia)