By the time those two make it to the final room, Rosi is definitely near the top as well - he'd been planning to assist with the heist the whole time, after all. He's definitely not mobile enough to keep up with mr. parkour out there, but he can sure help fight Cheri. He's got his pistol loaded, with a few improvements for capacity and stability on it thanks to Percy, and he can probably get at least one completely silenced shot in before Cheri notices he's even there. No squeaky floorboards here when you can just mute everything!

He also has a grenade, a couple basic shrapnel bombs and a smoke bomb in his possession, so if nothing else, he could toss a smoke bomb in if they end up just needing a bit of cover for an escape, but that would be a last resort if they can't incapacitate her somehow.