is it online coop or local coop

[Based Duffle Bag Boi]

thinking of buying the game

[Casual Amateur]

I 100% sure it is local coop. Of course you have to unlock the second player mode and you need to use 1 gem per day to play as Soleil every time as second player.

Getting those gem is pretty easy. You will see what I mean later on. But you can't play second player from day 1.

[Catalyst]

Sadly, it's local only.