Paradox

Paradox is Zac from the first timeline who went back in time in order to save Jax.

Rewind Arc
After Zac killed Jax in order to stop Chaos Samuel tells him that he can use his Elemento ability Rewind in order to send Zac into the past to stop this from happening, but he can't be certain when or where he will pop up. He accepts the risk because he knew Jax would do the same for him. Before he went he asked Micheal and Maxwell to build him a new right hand since his was destroyed in the Gold Blood. The two ended up creating the Chrono Gauntlet which had all the information that they had since the academy's creation. When he time traveled he ended up on the day September 22nd ★215.