Maxwell "Max" Tennyson is an alternate "what if" version of Max. He first appeared in Gwen 10.
In the beginning of Gwen 10, Max takes out some bikers. When Gwen was in the clutches of Vilgax, he jumps into the ship in the Rust Bucket with Ben. When the Omnitrix comes off of Gwen, it attaches to him. He turns into Upgrade, merges with the RV and escapes.