The tier list is actually pretty difficult for me to make up. To be honest I feel like this game is extremely well balanced and that all the fighters can equally be extremely powerful in the right hands. I've experienced competent and worthy adversaries that have played as everyone, even a good Helix which focused on trickery and trolling as its primary offense. I'll admit Helix might be at a slight disadvantage because of his default arms but in the full game he can use anything, and even then like I said even with default arms I saw people use Helix rather competitively, they even did the whole thing in the trailer where they became tall and evaded a super. Spring man is good overall and can counter, and I find he's more of a waiting type of playstyle. Ribbon girl is more mind games and evasiveness and speed. Ninjara is more of a fake out and movement guy as well. Master Mummy is a powerhouse and can destroy and bait people with ease. Min Min is agile and can block attacks, and the charged left arm is no joke. And mechanica's hover is surprisingly useful and tricky too.
Meanwhile they all have their downsides as well, I feel.
