Steven is in all manners the same as Gemini, with only a couple of minor differences:
The smaller part of Steven fires shots much more aggressively and is directly bound to the bigger part of Steven instead of via an umbilical cord.
After taking about a third of its life, the smaller part of Steven will separate from the bigger part of Steven to chase Isaac independently at high speed.
Upon death, each Steven will explode into several high speed shots, similar to a Red Boom Fly, but much faster.
Their HP is divided equally to each one.
The same tactics that work on Gemini will work on Steven, only care needs to be used to avoid the more rapid rate of shots. Upon the split, the player must be careful not to run into one Steven while trying to avoid the other.
Steven drops one of two items, Little Steve, a familiar with homing shots, or Steven (item), increasing damage and causing Isaac to grow a tumor with a face on his head. If the player already has one of these items, the other will drop. Despite the rumours, the order in which you kill the two Stevens has no impact on the item dropped.
Eternal Steven is much like Eternal Gemini. 2 little stevens are attached to the large one and will shoot 3 shots at Isaac. The first one comes off once the large one is at 2/3 HP, and the second one comes off once the large one is at 1/3 HP. All of them explode into 8-way bullets on death. As long as the large one is alive, all 3 of them slowly regenerate HP.
Steven is a reference to Edmund's game, Time Fcuk, in which a chronologically-displaced version of the player character finds a small person growing out of the side of his head, and names it Steven.
There is a connection to Time Fcuk and The Binding of Isaac, apparently beyond Steven's boss appearance, as noted in the Q&A for Time Fcuk in the Basement Collection.
From the original game, if Isaac was killed by Steven, his picture in 'Isaac's last will' will be of Gemini.
One of the messages from a Fortune Telling Machine is "Steven Lives", referencing to Time Fcuk when Steven says "Steven Lives".
When using the Mom's Contacts item on Little Steven, he'll freeze in place, and suddenly teleport if he's still connected to Big Steven. Usually he'll teleport next to the player.