Judas is a character unlocked after beating Satan for the first time. Judas starts with one red heart container, but has The Book of Belial upon starting, as well as 3 coins.
When a character dies after picking up the item Judas' Shadow , they will respawn as Dark Judas. Dark Judas spawns with two Black Hearts, a damage multiplier of 2.00 (instead of Judas' 1.35 multiplier), and a base speed of 1.1. Similar to Azazel, Dark Judas can still gain heart containers despite starting with none. Dark Judas has the highest base damage multiplier of all the playable characters.
Dark Judas is considered by the game to be a separate character from Judas, but all of the Judas Completion Marks on Judas' post-it note can be completed with Dark Judas and unlockable items and/or achievements can be earned by completing objectives with Dark Judas.
- Judas has a great damage sustain during the run due to the The Book of Belial ; try to keep the item until you have enough damage to grab another active item.
- Accept any health upgrade during the run, as Judas can be challenging without enough health. Grabbing items such as Bucket of Lard early in the game could be crucial for a good run.
- Scapular and other "low health activation" items are helpful for early floors, due to his low health.
- Dead Cat can prove very useful if found early enough, since Judas starts with only one heart container. As well as granting 9 additional lives, it is also one of the Guppy items.
- Judas is a reference to Judas Iscariot, the Thirteenth Disciple. Judas betrayed Jesus to the Romans in exchange for thirty pieces of silver, represented by the three coins Judas starts the game with. Moreover, the coins are also believed to represent the three Abrahamic faiths, Judaism, Christianity, and Islam.
- Contrary to popular belief, Judas's fez is not a reference to the game Fez, created by Phil Fish. Edmund Mcmillen has stated that the initial statement was a joke and that he used the fez due to it being easily recognizable.