Cool game idea, I really enjoyed playing it. Nice controls and I like the level up system. Two problems though.
Small problem : I think the character art is backward.
Big problem : the game becomes easier the more characters you have. By the end you can just stay still and spam click, and you gain enough xp to regenerate characters faster than they die. Possibly make the amount of XP required higher based on the amount of characters you currently have, or make the enemies harder to kill.