Hmm... it's hard to imagine that Cloud's "dearest love" wouldn't be a person. Unless he has an extreme liking for his hair or something.

The secret things you've been hiding

There are lots of "secret" and "hidden" things in this game. Cait Sith hides the fact that he's a spy but he's hardly Cloud's dearest love (unless he 's weirder than we thought).

Is it maybe the White Materia? It is hidden that Aeris has used it and prayed for Holy. Plus, it reminds Cloud of Aeris "lost in memory"

Actually, that's pretty thin. I don't think it's that.

I've no idea. The riddle seems a bit too vague unless I'm missing something really obvious which I probably am.