Standard (non steam) characters can play on pc, ps4, or ps3 (you just need to buy each version of the game once and register it to your account) but they cannot log into steam. Hope this helps.
Just to clarify, I believe a non-steam account can add the steam version to it. I started playing on ps3 and just bought the game on steam and had no issues registering it to my account. I think the restriction is on allowing the Steam version to play on other platforms.