In Co-op mode, Arthur's stats and passive do not change.
Arthur's main Hyper, Branch Expansion Strategy, does not visibly change in Co-op.
Arthur's secondary Hyper, Rbit Hobby Shop, has a slightly different effect in Co-op. The Hyper has no effect on enemies, and no stars are gained or lost when triggering this Hyper. (In all other aspects, this Hyper functions similarly to its Normal mode counterpart.)
Arthur's passive and Hyper are unchanged in Bounty Hunt mode.
Arthur's base stats give him 44 starting stars and a discount of -10% at shops.