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I suppose the illusion of teleportation is because he vanishes downwards, and then seemingly appears anywhere he wants. Nobody knows where he's going to pop out next, you have to know that beforehand and be watching that spot to even have a chance to see it clearly. The fact that he has to fully submerge before reappearing is a pretty huge clue, but Class A didn't have the time for in depth analysis, and they didn't even know about the "pop up" function of his Quirk before he explained it to them (Ashido even thought he had two Quirks). I'm guessing he can orient himself while submerged and so not give away his intended location while on the surface (the diagram appears to suggest as much).

Wow, if he had been given One for All he'd be super unstoppable.

And of course All Might has a former sidekick we're only hearing about now.

Glad to see some follow up from Double's spotlight chapter.