Yeah I didn't even notice Rook until my second reading. Considering the massive backstory/buildup all the other combiners have gotten in IDW (Mons, Dev, Men & Sup) for Defensor to turn up as a matter of convenience felt too easy and rushed.
That said, it was an interesting last panel where you had Starascream looking pleased as punch with Defensor and Superion standing behind him, ready to do his bidding![]()