i'm gonna go with kuroka, she hasn't been mentioned a whole lot since volume 15, i think she starts to develop deeper feelings for issei, at some point she realizes how she feels, and before you know it, issei wakes up one day and finds a nude kuroka next to him (good times). i think a big factor in this could be issei fixing kuroka and koneko's relationship to the point where koneko fully accepts kuroka as part of issei's harem, then i imagine she'll ask issei to take care of kuroka.
- on a side note, i have a question that i've been thinking about, is kuroka a virgin? the way she talks about having sex with issei you would think she's already had sex, but if she is indeed going to be part of issei's harem (and i think she is), wouldn't the fact that she's not a virgin be a bit of a head scratcher? why break that virgin pattern now of all times and why break it with her?