I could imagine it being a third person open world action game. Something in the style of stuff like Infamous or the Spider-man PS4 game, where you're running around the city doing quests. You get alerted of demon attacks and rush over there, or then there might be plot things that happen at points. Maybe you'd play as Rekki and mainly follow the main plot, or perhaps switch between say Rekki, Sarin and Jules, or if it'd be a new character with a new plot. I personally feel like people would rather rampage around a city as Rekki Lune than play somebody unknown, and the main plot would translate pretty well because the prologue introduces all the basic mechanics in a context where your powers are limited and you're getting advice from a mentor figure.
The gameplay of an Errant game is pretty easy to think of, like your sword would charge up while it's sheathed, and when you draw it. you can launch a powerful draw attack. If your sword is unsheathed, you'd fight with your sword, giving a boost to how well you damage demon bodies, but reducing the power of some of your other abilities, so you'd need to think tactically, just like the characters in the comic do, about when to draw your sword.
...Damn... it's a shame such a game would require way more budget than I possibly have because leaping around Fantasy Manchester with super-strength, punching big demons so they explode into clouds of rainbow dust and then having silly interactions with friends actually sounds pretty fun. 
