I believe you only get to see Lucrecia if you have gotten Vincent. You see her in some of his flashbacks and in a secret cave that can only be accessed with a submarine or gold chocobo (maybe black too).
[The Truth]
Warning! This section explains nearly everything about Lucrecia. If you don't want to know, then don't read!
Lucrecia is Seph's mommy, Hojo being the daddy (yuck..........). She is stuck in this cave for some reason, kind of as a ghost but not really @_@. The whole story thingy was that when Vincent worked as a Turk, he liked Lucrecia. I guess she must have either liked Hojo or he was raping her (ewwww....) because they had sex and Hojo put jenova cells in her. I forget what happens (long time ago) but Vincent tries to shoot Hojo (which doesn't work) and gets caught by him, then turned into what he is now.
