When Rukia is gone, mysteriously swept away by that most distinguished of noble houses, the Kuchikis, Abarai isn't the only one who misses her. True, Momo and Rukia had never been close, but without her around, Momo is missing two things: a buffer against her attraction to Abarai, and a concrete reason not to pursue him.
Only fear holds her back in the end. Mostly that even if he didn't reject her outright, he'd be unable to keep from comparing her to Rukia. It's a realistic fear, Rukia is all he talks about, thinks about, lives for. Momo only wishes that someone could love her so much, that when she was gone every aspect of their being changed.
For a time, Abarai was in a strange sort of mourning. Alternately cursing his perception of his own inadequacy, and the system of nobles that forced the situation on him in the first place. With enough sake in him he could get quite violent, lashing out at anyone who got close enough to make eye contact. She and Kira always stayed with him, talked him down and occasionally beat some sense into him. They were the elite of their class, they had to provide a good example.
Occasionally when he was like that, she couldn't even understand what she'd ever seen in him. Then for a few days he'd get his swagger back, challenge her or Kira to one of his stupid competitions. Or he'd just sit with her and talk about anything, kido, history, the unfairness of loving someone you couldn't have. When he tried to talk of love, she always made an excuse to leave, before she said something foolish.
A year later he was virtually unrecognizable from the bumbling show-off she'd first met and befriended. It was not just the tattoos (and even if she could only see a few of them, she knows he keeps getting new ones). The fire that burns within him only seems to intensify with each passing day and his potential grows with it. He studies and practices and pushes and drives himself like the gates of Hell are opening behind him trying to drag him in. Watching him only makes her want to do better, too.
Her feelings for him crystallized at some point. Where before she'd only felt fondness like that she had for Kira, there appeared admiration and affection. It was different from the crush she had on Captain Aizen, which she acknowledged and they both teased her about. Momo doesn't have fantasies about kissing the Captain behind the dojo, or draw hearts around anyone else's picture in the "50 Most Up-And-Coming Shinigami" article in the Seireitei Communication. There isn't anyone else she goes out of her way to watch practice, no one else who she has dreamed about making love to her under the stars.
Toshiro is the only one who guesses her secret, though she has no idea what clued him off. He calls her a fool and Abarai an useless idiot. It doesn't matter. She knows it's hopeless, a fancy, and that in his single-minded pursuit of power and status, she could only become an obstacle. Momo Hinamori is not and never will be Rukia Kuchiki.
It doesn't matter, so she'll never tell.