Surely not at every point in the game. From what I've read the early missions entail 'unlocking' i.e. rescuing the 3 unselected characters i.e. if Lucas you need to rescue Angel, Gun Girl & the Alchemist (okay nicknames may not be accurate) before they can join your party. In Co-Op people play as your allies, which you won't have to begin with. That means no Co-Op until you're partway through the game right?
It is said that your friend can play any of the other 3 characters, even if they are locked for you (as a host player). Let me quote Co-optimus:
Ah, my mistake. I've only read a couple of the previews so far and hadn't seen that. I'm sortof hung up on the AtomicGamer one that describes DS3 as being akin to Street Fighter, regular attacks being used to power up the ability to launch an uber attack. Oh and another one that said no regeneration. Mana's out I understand but I anticipated health regen. Apparently that's not the case. Lucas Skywalker apparently has a regen stance but whether that's common to all characters ???
All characters have a defensive stance like Lucas.