Following...

This REALLY shouldn't be a hard question to answer devs. Considering I bought this game to eventually co-op with a friend and can no longer refund I would really like to know how to get online co-op to work.

Basically, you only want to join someone else's game when you're between chapters. Joining in the middle of a chapter means that you will lose your mid-chapter progress unless you beat the chapter. It's a boneheaded system.

Agreed, but I can't even get to join the game after I decide to lose my progress.

Are there random people that will join your game like in RCR underground? Or do you have to know someone to play?

Yeah it's strange overall. Online works a bit more like how you'd expect where your own stats, items, and moves carry over to your partner's story progress, though for whatever reason they chose to make it so locally you're always bound to the same thing, which even the other co-op Kunio games during the Technos days supported fine.

It's a shame cause I actually really like both this and Rival Showdown on the 3DS, which this game's gameplay & animations pretty much comes from, but I'm really hoping ARC puts a better focus on co-op play if they ever choose to make even more traditional Kunio games in the future