Hi! Developer here! I'll try to answer as concisely as I can

1- You hold back to block
2- The game is single-character. A long time ago there was some primitive tag functionality implemented, but ultimately I felt like designing and balancing the game around a tag system was too complex from a development standpoint so I ended up removing it.

3- There are motion inputs for special attacks and supers

4- The game's movement is more "fluid", as it leans in on air movement techniques like airjumps and dashes

5- Costumes are a possibility but not a guarantee. I've had help dabbling in them before but they require a full re-sprite of every animation frame so they would be a pretty ambitious addition. I would like to include them if I can but they aren't on the immediate future agenda.
6- Its a traditional fighter

I hope this helps!