I got to lab a few more characters today to see how they mesh with Ultron.
Rocket - This guy is legit good, and his hit box makes him a cut better than a lot of the cast if you haven't practiced Rocket specific combos. Throw in that space stone with the traps, and holy hell he is a mix up machine. The synergy between him and Ultron is non existent though. Rocket just seems to do his own damn thing, and in that way he can go with everyone in the roster, but not in a fully effective way.
Iron Man - Shiiiiit. Tony isn't stupid hard to play for once in a Marvel game. I hate that forward HP move even existing though. I'm too used to do run up dash HP as Ultron, and too often with tony I get a rocket. The synergy is here though between these two, and they play quite similar. Easy ass 6k+ combos full or corner is making me dig him with Ultron.
Captain Marvel - wowowowow. This character is TOO damn good. I spent most of my lab time with her once I got on her. Her mix up game if she rushes you down is fast and mean. Jump down + HP, air dash LK is hella hard to block effectively and then crosses up. If she has you corner she can do Jump down HP into double LK. Her damage is great even without the buff up super mode, and her combos are just hella satisfying to do. The synergy is ok with Ultron. Their supers combined aren't anything to write home about. You are better off just sticking to the 6.5K 1 meter damage combos with her and Ultron and keep the extra meter to throw out a full screen beam super to then tag in and get some high low mix ups in while it locks them down.
I hit a new record for damage with a little over 8.5K using Iron man and Ultron. Took all 4 bars of meter, and no stone or storm use. Regular ol combo with Ultron into an air super, then as they fall down you tag in Tony for that level 3.
Edit : @Bro above me: Are you on stick or pad? I feel the control system in this game was made for pad players in mind a lot. I actually am glad because my legacy stick support isn't here, and I can do all my combos on a pad in a Marvel game. I can't wave dash effectively, but idgaf. I'm not going pro so it's all good.