Mitty

Mitty was an amorphous narehate that lived with Nanachi on the 4th layer of the Abyss. Mitty had been a regular human girl in the past, and her appearance changed due to the result of the ascension from the 6th layer.

Appearance
As a human, Mitty was an upbeat and positive young girl. She was almost always smiling and aspired to be "a future White Whistle." Her hair was reddish in color, long, and messy, sticking out at random angles. Some of it seemed to be tied up into a messy ponytail on the back of her head. She wore a tied top and skirt. She also appeared to be slightly more buxom than the children her age.

After bearing the Curse of the 6th layer, Mitty's body became something of an amorphous blob. She became covered in light pinkish fur. On the ends of what was left of her arms were long sharp claws. She had two catlike ears protruding at odds angles from her head. Her mouth split upwards and was left hanging open at all times. Both her eyes bulged outwards, until one of them was hit with Bondrewd's Relic Sparagmos and failed to regenerate.

Personality
As a human, Mitty wanted to become a White Whistle. She was very upbeat and positive, wondered endlessly about the world around her, and always tried to look on the bright side of things, even if Nanachi was often negative. She was very outgoing, as she was the one who first approached Nanachi and insisted on being friends and partners.

After becoming a narehate, her intelligence vanished, and she could no longer communicate with Nanachi. All she could do was to make soft noises and crawl around a bit. When hurt, she cried reflexively, but likely didn't feel any physical sensation nor emotion; as Nanachi said, Mitty's soul was "trapped inside" her narehate form.

Dreadful Experimentation Flashback Arc
As a human, Mitty was brought down to the 5th layer by Bondrewd along with other children. During the descent, she met Nanachi and the two became close friends.

This was unfortunately short-lived, as once they arrived at Idofront, Bondrewd used them as test subjects for his experiment in warding the Curse: Nanachi and Mitty were put in an elevator that lowered them to the Miniature Garden of Dawn in the 6th layer and then rapidly forced them to ascend. The capsules they were in transferred Nanachi's end of the Curse onto Mitty, leaving Nanachi unharmed. Mitty had remained positive until the Curse took effect, insisting she would survive, and even attempting to calm Nanachi down by repeating that it would be okay. Once the Curse began affecting her, her words became screams of pain and pleads for death. The experiment resulted in Mitty's body deforming beyond recognition and her intelligence vanishing along with it.

Bondrewd continued to experiment on her and found that she was virtually indestructible. Unable to bear Mitty's suffering any longer, Nanachi took Mitty and fled to the 4th layer, where Nanachi constructed a hideout for them to live. During this time, Nanachi tried numerous times to put Mitty out of her misery, but nothing could finish her off.

Poison and Soul Liberation Arc
After Nanachi sent Reg to collect the ingredients needed to help Riko, Mitty spent a lot time near the unconscious Riko. When Nanachi observes Reg's Incinerator, during the second encounter with the Orb Piercer, they are surprised that it seems very similar to Bondrewd's relic Sparagmos and realizes they may finally found the way to free Mitty from her suffering. After the the fight is concluded and Reg asks how he can ever repay Nanachi for all their help they ask him a for a favor; to kill Mitty and set her free.

Reg, first shocked by such an request, reluctantly agrees; after having given it some thought, but only if Nanachi promises they won't take their own life afterwards out of sorrow. Nanachi agrees and they prepare an final resting spot for Mitty, surrounded with stuffed animals. Reg uses his Incinerator on her that she finally dies. Nanachi bursts out in tears and mourns the loss of their friend, while being comforted by Reg. Later on, Riko reveals, while she was unconscious and her mind was tortured by the poison, she felt the presence of another girl being even more unfortunate than she was and they comfortated each other with just sensing each others presences.

Just before Riko woke up she felt the presence of the other girl, being Mitty, moving on.

Ilblu Arc
Despite her supposed death, a Mitty replica is now seen to reside in Ilblu, where it is said she was left behind by a "masked man." Belafu smelled the original Mitty's true immortality and craved it so much that he gave up large portions of his body for her to be "reborn/replicated" into Ilblu. Since then, Belafu has used her as his main source of food, saying that Mitty never runs out, no matter how much of her is eaten. This one is actually just an earlier replica of the original Mitty.

Later on, Majikajya reveals to Nanachi, in a conversation about value, that one of the Three Sages of the village, Belafu, is in possession of the supposed-to-be-dead Mitty. Nanachi asks Majikajya to take them to the sage.

Later on, Riko visits the sage since she has been informed to find Nanachi with him. Riko finds Nanachi and Mitty with Belafu, and he tells her how he attained the Mitty replica. In a short flashback, it's shown how Nanachi is moved to tears out of happiness to be reunited with Mitty, and how they are shocked moments later as they watch how Belafu extends his feelers to drink from Mitty. Belafu tells Riko that Nanachi offered themself up to him in exchange for ownership of Mitty so that Belafu couldn't drink from her anymore, and that he is willing to sell Nanachi back to Riko if she can give him something of similar value.

Immortality
After becoming a narehate, Mitty was resistant to every form of Bondrewd's torture and experimentation. She'd had stakes driven through her and limbs chopped off, but they regenerated every time, with limbs growing back in place of those that were lost, and all poisons and venom neutralized. The only known limit to this ability was recounted by Nanachi, who saw Bondrewd using Sparagmos (equal to Reg's Incinerator) on narehate Mitty. Mitty's eye was hit by the light and could not regenerate anymore. Nanachi tried everything they could to put her out of her misery during their time together. In the end, the only thing that was ultimately able to kill her was Reg's Incinerator.