After reading over the chapters again, I've got a feeling that the girl in that photo was Yotsuba. Or at least , Yotsuba knows that they've met before. She was the first person to stick with him after he became the tutor. After that chapter when she "lied" to him, I think it's her. Or at least I want it to be her, lol. This feels like the whole Love Hina thing all over again except there wasn't a confession.