uMutantheart is an optional mini-boss enemy that can be fought inside Humphrey in the hikikomori route. She can be found standing inside Marina's room with her creator. She bears the image of Sweetheart, but made of a jelly substance.
Like most enemies, she will act differently according to her emotion:
In addition, she will ask every turn for a specific emotion, if a party member is not affected by this emotion by the end of the next turn, she will "instakill" it, dealing 999 damage.
Skills that can be used when observe is used against the target are marked with .
Wink: turn a target happy.
Cry: turn a target sad.
Insult: turn a target angry.
Instakill:  damage to the user, this damage can't hit right in the heart and has no variation.
* Mutantheart winks at [target]! It was kind of cute... * Mutantheart accidentally says something mean. * Mutantheart slaps [target]! * Tears well up in Mutantheart's eyes.
Start of battle
* Mutantheart: H... Henno...
- asking for happy:
* Mutantheart: Happy... please!
- asking for sad:
* Mutantheart: Sad... please!
- asking for angry:
* Mutantheart: Angry... please!
- If someone's not on the right emotion:
* Mutantheart: Bleh... Wrong!
- 50% :
* Mutantheart: Bluh...? * Mutantheart:
* Mutantheart: Bloooohhhh...
When defeating the player
* Mutantheart: Bleh!
* Mutantheart sings a song for [target]! It was not the best... * Mutantheart strikes a pose! * Mutantheart fixes her dress! * Mutantheart fixes her hair. * Mutantheart: Bleh...?
- While you can't fight her during the true route, she is seen being created in one of the cutscenes with Sweetheart and her creator.
- As seen on the formulas, the 'Instakill' action is not made by Mutantheart, instead the party member applies it to themself, killing themself.
- Her "cry" action uses the same text as Lili's 'cry' action.
- The unused "attack" skill would use this action text.
- Its unused "do nothing" action text.
- The unused "heal" skill would use this action text, it would heal the user by 250 heart.
- would be used before a "strong attack", likely a scrapped text for the "instakill" move.