Oh gods, she would *definitely* have that effect on Sephiroth. She'd just look at him and be all big eyes and complete uncomprehending clueless cuteness, and he'd start winding up with a headache just from being in the same area for too long. Tohru Honda should clearly come with a warning label for villainous types - "Warning: Prolonged exposure may cause unjustifiably heroic actions. Use only under controlled conditions. Not safe for minions."
Mind you, I'm not sure whether having her in the same area as Aerith is going to be a good move. Then again, the worlds of KH happen to be fairly resilient - after all, Hollow Bastion can handle both Cloud-angst and Leon-angst, and still cope with Seph-crazy-angst, so I figure they're a little less subject to catastrophic total reality failures than most. Of course, Hollow Bastion/Radiant Garden has just gained another anti-Heartless defence - if they come near Tohru, she'll wind up feeding them their favourite food and just cuting them into submission.
I wonder what would happen to her in the World That Never Was... I have this horrible suspicion she'd soon have the lesser Nobodies following her around like lost puppies, while the members of Organisation XIII looked on in vaguely stunned horror. Except, of course, for Demyx, who'd just shrug and let her feed him and look after him and stop him from having to fight anything nasty.