As Yuffie's housemate who she hates I'll throw you a line as discussed - feel free to go into his room when he's not there and steal things. She already stole his useless materia that he'd stashed in the bottom drawer!

Though, beware of going in there when he's in the shower or downstairs eating or some such; Spencer will probably give her a telling off and just loud enough for Sephiroth to hear.