Sorry to repeat myself, but please increase the helpers time on screen. Some of them come and go away and you may not even notice it. Some could just be slowed down a little (like Firestar), others could taunt after attacking, or just make a "big" entrance (like when Mugen fight starts). Maybe some of them should come in and out multiple times, from different sides, or jump/teleport to another position and attack again.
In some cases, this can be done.
Emma and Jean's mind control could be made more visible (enemies could change colors or have an icon on their heads, or maybe the helpers could stay on screen as long as the effect is working, like a psychic projection).
This issue has relationship with how OpenBOR spawns flash/hitsparks. Unlike Mugen, they are placed automatically and I never figured how...until I made a move with a big attackbox. Seams that the engine uses the attackbox boundary to place the effects, render the move to spawn the flash/hitspark outside of the screen. This way is wondeful in 95% of the cases, but can be frustating in cases like mine.
So, I will code it using script, to make the hitsparks to appear. And on the mind control case, my idea originally was to make the enemy to change its color when its mind controlled.
Storm's cape needs an animation, badly. Sorry
Its beyond my spriting skills :-\
Firestar could leave a trail of fire on the ground.
Firestar fireballs are strange: they remove itselves once they touch the ground, and they should not do this. But I like your idea!
Before leaving, couldn't Captain America do a "pep talk" or something? Give a big speech to increase Morale among the troops - I know giving an Attack bonus may be too hard to code (possibly waste of time), but maybe healing could work too...
Hum, it could works...
Overall, I quite like them. Some seem really weak, while Thor is... well, a GOD!
There's a lot of power differences between helpers - which is good! But I'd feel cheated if I called the Avengers and all they sent me was Hawkeye.
You got Thor right

Yeah, the helpers power are intended to be different from each one. About Hawkeye, don't dismiss him: His accuracy is only surpassed by people like Bullseye and Boomerang. Take a look at the video bellow (2:05) and you will see what I am talking about
Avengers Earth's Mightiest Hero: Hawkeye
Maybe I could make his helper version with homming arrows?
Random idea: how about a helper who stays on screen building something or concentrating on a spell/power and the player must protect him until it's finished. If an enemy hits the helper then the effect is minor, but if he's able to finish the casting/construction then the effect is much more amazing! For example, Beast can bring his Robot helper, but if he has time to work on it, then he brings a Sentinel. You could apply this to Storm or Thor (he could stay in middle of screen whirlling his hammer, and the longer he can do that without interruption, the greater the damage done).
I had the same idea, but requires more work thanks to the way I made the helpers: they are a single entity, which has all the frames of all helpers, and each helper only uses one animation (with exception of Storm). This helps me to not make 87687123123123 new entities, to load them and call them, but can make some things a little harder. So this is why I've choosen to make all the helpers invencible.
Thanks for all your feedback
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Talking about Tigra, she doesn't has a running animation, she does short hops. The hops are fine, but since she is a feline, she would need to RUN as feline. But I need help with that.
Any good soul to help me?