Despite what some of the people here are saying, the green guys CAN be killed... but, only if you were already shooting at their lane when they spawned. They are impossible to react to. Which, is a problem since they so often appear alongside 2 spike walls; if you weren't lucky enough to already be shooting at them when that happened, whoops take damage now, no way to avoid it!
Anyway, I am unclear on what unlocks more characters... after the first one (Alice), I got two more (Nichole and Sasha), but no idea why. Both are objectively just plain worse than Alice, though; both of them have less health (which, is really the only important stat), though Nichole has a double shot that makes her hit harder it is too slow to be worth it, and Sasha... I am not sure what her shuriken are supposed to be doing, they seem to move in weird ways after hitting an enemy, but so what?
i comepletely agree with all of this!
i would also like to add that the upgrades arent really upgrades! even at level 3, the first monster takes 4 shots, same as level 1. so what is the use? a good way to upgrade is the first monsters takes 3 instead of 4 shots at level 2, then 2 shots at level 4.
I like where this is going and if you want more such things, let me know, i am glad to help 'for free' with ideas and such to make the progression feel better.