What could be his secret? Evil twins?

Not twins. Remember the previous chapter and how they said there was a failure? I think the failure is Haru and Ao (blue) is the perfection.

However, Ao didn't like the way they treated his brother so he destroyed them all. Sounds like he's a psychopath and latched on to Haru.

Haru seems to have used their shared power to erase his memory to protect other people from Ao.

I think Ao was the torn apart being from chapter 3. I wonder if Haru did it? To protect the others?