User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23853100-20131019035942/@comment-5185939-20131019181620

Yasaka is a potential because she's a named female, and Issei directly saved her and her daughter, so that suggests there is hope there.

Valerie is a potential because she's another named female, though I almost wonder if she'll end up being Gasper's love interest instead.

Basically, if they're a named female, alive, involved in the supernatural, and not stated as already being interested in someone, I'm considering them a potential.