I was afraid of the game becoming too easy if I added a healthbar, but from playtesting it seems I might've made game too hard - because I am myself pretty good at platforming, I didn't notice it (and I didn't really have time for playtesting and iteration because it's a jam game). Worse, I was afraid that having two separate healthbars for each character might be too complex (I didn't think of the shared healthbar idea). If this game gets into top 100 (doubtful) I might make a bigger, more polished version in Unity or Godot.
EDIT: Wait, I have an idea on how I could represent the health without a healthbar. I could palette swap the characters so they change color and brightness (brightness to be fair to colorblind players) when hit.