It's a pretty fun 2D beat-em-up made by the dev of HoloCure, with contributions from others in the fandom. It's the latest Steam release under holo Indie.

Proper online mode had to be delayed, so its currently single player, but there's an option to use Steam Remote Play to play with friends that way

Works fine for me on Fedora Linux with Proton Experimental

Game Description from Steam:

Fight your way through endless waves of holoX's minions! Gear up with all sorts of equipment and items in this action packed beat-em-up to reach the 5 bosses of hololive's "holoX", and put a stop to their plans!