The Mask of Infamy appears as a huge, disembodied mask-like face with empty eye-sockets, a sad-looking expression, bloody tears streaming from her left eye-socket, and a screaming mouth with no visible teeth. She sports with a heart from behind.
During the second stage after defeating her heart, her face changes into a furious look. Both of her eye-sockets are also streaming more with bloody tears.
During the first stage, the boss consists of a Heart and a Mask - only the Heart will take damage. Both the mask and the heart float around the room, the mask moving faster. The heart occasionally shoots an 8-way spread of bullets. The mask charges after the player, similarly to the Selfless Knight.
During the second stage, the Mask's face has changed and become more aggressive. During this phase, the heart is destroyed and the face can take damage through the small hole in the back of the mask. The character must attack the side or backside of the mask to damage it, unless they have piercing weapons (similar to a Knight). Neither stage has a health meter.
As of v1.33, an AI buff to knights makes Mask of Infamy incredibly difficult to defeat without piercing or crowd control. The Mask makes 180 degree turns much more often, and nearly instantly will turn to face and charge the player if they attempt to approach from behind. However, it is possible to defeat the Mask with the use of poisoning items such as the Pinky Eye if the player can evade it long enough to drain its health. Another useful item would be The Pinking Shears as the separate body will follow closely behind at all times and makes for a quick fight. Another good tactic against the Mask of Infamy is to use an ability that slows down the mask (such as Spider's Bite or Little Gish) which can help you to get shots at its back. An effective, but risky, strategy is the use of bombs. When the mask charges at the character, they can drop a bomb and hopefully the mask will be close enough to the bomb to receive damage. If nothing else, speed is necessary to win the fight. Charging along perpendicularly to the Mask and shooting tears will damage it but the Mask won't charge. Focus on shooting as many tears as possible and being fast and the fight will be long, but possible.
The heart fires homing shots in a plus (+) pattern.
The Eternal version of the heart shoots 8-way bullets 3 times in a rapid session. It will split into 2 small Eternal Hearts on death.
The Eternal Mask, however, acts exactly the same as its normal counterpart.
Strangely, there seems to be a face in the back of its mask in the second phase of the battle. Any information on this is unknown.
In Rebirth, The Gate is also known to have this figure hiding inside him.