The big mystery is who the girl in the foreground is...

Based on the size of the ass in question, I'd say its Kokoro or whatever the hell the new girl's name is. Christie and Helena's seem larger to me and Tina's ass is like twice that size.

None of the DoA girls have backsides worth noting. They range from flat as a board (Kasumi) to barely curved (Helena) with a barren landscape in between.

Rachel has an ass, but she has yet to appear in a DoA game. I also think it's odd that they'd put Rio from Super Blackjack in DoA Paradise and leave Rachel, Sonia, and Momiji, but hey.