Yeah Birth Prototype was my first one. I thought you just use the weaker ones to power up the ones you want.

Don't I need to match the attack type? Blue gun, green fist, etc?

Edit: Whoa. I got a Gaim somewhere.

Anyone looking to play this or has it and needs help this Facebook group is helping with translating the screen to help guide you:

https://www.facebook.com/KRStormHeroes

Sometimes I wish they just ported this to mobile:

https://www.youtube.com/watch?v=b87HM_50caY

There's also the Wikia, though they haven't updated in a while and it's missing a good deal of info. Still, it's a decent starting point.