Supposedly level 0 espers have no powers at all. Are they any different to humans then? Are they supposed to have latent powers or something?
Level 0s have undergone the Power Curriculum Program, so even if someone ranked at Level 0 doesn't happen to exhibit any observable powers, their brains have still been fundamentally rewired. All espers, including Level 0s, are unable to use magic without suffering severe backlash damage because of incompatibility issues.
But it's not necessarily the case that Level 0s have no power at all. Level 0 is described as follows:
Exceptions aside, cannot be considered completely powerless, but is considered one of the so-called "students that can't keep up at school". Might possess some degree of power, but unable to truly control it.
- Tsuchimikado Motoharu has a Level 0 Auto-Rebirth ability, which applies a weak membrane over torn blood vessels to stop bleeding. It's a pretty weak ability, all things considered, but it's enough save his life from the backlash damage of using magic.
- Akemi has a Level 0 Telepathy ability, and she can levitate a paper cup (barely). At the end of Railgun episode 14, she and her friends remark that their stats have gone up a little, even though their levels haven't changed.
- Saten Ruiko has a Level 0 Aero hand ability. This is her System Scan result from Railgun episode 14:
As you can see, it's not all completely zero. There's still some amount of power at Level 0; enough for Saten and her friends to be able to see some improvement, at least.