D-13

D-13 is the second playable ship in the game. Its capabilities are equal to those of Null, the difference between the two ships being that Enemies and Bosses are more powerful while playing as D-13, often with more complex attacks, new phases. D-13's Unknown Boss is Monolith. Like Null, its default weapon is the Peashooter.

Once floor 6 has been cleared with D-13, the "Hard Mode" aspect of D-13 is separated from the actual ship and now controlled by a separate toggle, allowing any ship to be played on any difficulty. This includes playing D-13 on normal, meaning its differences from Null become entirely cosmetic. Whether or not monolith is the boss of Unknown is controlled by Hard Mode, and will override any unique versions of Machine characters like Overlord and Chaos have.

Story
D-13 is the 13th Visitor of the Facility, and enters with their rival Null. Their progression through the Facility has them tailing Null, encountering more deadly versions of each boss, and finally catching up with Null at the Power Eternal conduit.

Upon meeting up with them, Null proceeds to transform into Monolith and begin its assault on them.

After defeating Monolith, D-13 destroys the conduit and exits the Facility, landing on a hillside beside a now broken Null.

Trivia

 * Team D-13 gets their name from D-13 the ship.
 * D-13 shows up in The Binding of Isaac: Repentance as a co-op baby.