Tifa has a lot of thoughts and concerns all bottled up. She's a champion bottler. For now, she's just focusing on trying to figure Zack out. Back in Nibelheim, she'd thought he was part of the problem, being SOLDIER and thus Shinra. With all of the perspective she now has, and all of the memory confusion with Cloud, she's trying to put some distance between herself and everything. She still has very strong feelings against Shinra, based on Nibelheim and the Mako Reactors, but Zack is something of an anomaly now. Not responsible for Nibelheim, possibly tried to stop Sephiroth, but still... SOLDIER. And yet Cloud's been calling himself SOLDIER, too.

She's confused on a lot of things. At least she's giving Zack a chance here.

"Thanks," she says in response to keeping an eye on Sephiroth. "I saw him once already and we... kinda agreed to stay away from each other. I just have to make sure Cloud does that." Or at least doesn't antagonize him.