Mitsuba Sousuke/Synopsis

Mitsuba Arc
Mitsuba was first introduced through a rumor of a ghost that haunts the middle school lockers, grabbing people as they change their shoes and asking them "Do you remember me?". Kou caught him on the act when he grabbed his arm, and after consulting with Hanako, Kou decided to help him finish his unfinished business so he could move on from the world of the living.

Reach Out Your Hand
Mitsuba appears at the very end of the chapter.

Hell of Mirrors Arc
Mitsuba appears after the disembodied hands dragged Nene into the 3rd School Mystery's boundary. The two progress through the Hell of Mirrors together and attempt to find a way to exit or find the Mystery's yorishiro. Mitsuba jeers at and teases Nene, but since she agrees to help him out and is nice to him, he seems to like her. After Tsukasa appears and defeats the Mystery, he attempts to force Mitsuba to consume its heart in order to become stronger. Mitsuba, clearly upset and scared by this, attempts to escape from Tsukasa until Nene tells Tsukasa to stop. Eventually, he does but end up consuming the heart, and assumes the role of the 3rd Mystery, gaining a new outfit and enhanced abilities.

When Kou and Hanako enter the area, Kou is shocked to see his former friend, and attempts to speak with him. However, Mitsuba does not remember Kou, and it is revealed that the Mitsuba in the Hell of Mirrors was created by Tsukasa from part of the old Mitsuba's soul. As a result, the new Mitsuba does not remember his life as a human or his time spent as a ghost. He does, however, have the same personality and even refers to Kou by the same mocking nickname as he did before. Tsukasa tells Mitsuba that he can do anything he wants as a school mystery, and so Mitsuba wishes to become a real human and live a normal life. However, Hanako dissuades him from this, since neither ghosts nor created supernaturals can be allowed to live as humans.

The Melancholy of the New No.3
This chapter follows Mitsuba in the aftermath of his encounter with Hanako and the others in the Hell of Mirrors.

Picture Perfect Arc
Mitsuba appears as a normal classmate of Kou's, as if his death back in his 2nd year had never happened.

Later, it's revealed that the fake world's version of Mitsuba is in fact from the real world, having been brought to this world by Mei (and indirectly Tsukasa) in order to grant his wish to live as a human. We can see through his actions of compliance with Tsukasa, Mitsuba's desire to become human takes precedence over his self awareness. We can discover further depth of his character and a seriousness behind his outward projection of playfulness and teasing. He acknowledges that Tsukasa "doesn't care (for him) at all" yet knowingly clings onto him as Tsukasa is his only source that can grant his happiness. Mitsuba reveals that he intended for Kou (and Nene) to lose their memories of the real world as it is such, the longer they are in the fake world. He displays a rapid change in personality to a rather sinister demeanour revealing that whether the world Mitsuba lives in is fake or real, he does not care, and prefers to play a fabricated version of "Mitsuba" while ultimately lying to one of his closest (if not, his closest) friend, Minamoto Kou, for the sake of his wish (to become a human). Shifts in personality is a common occurrence for Mitsuba, yet his display in the Picture Perfect Arc could suggest emotional instability due to the trauma of his bullying in elementary school and/or the fragility of the Mei's world.

Mitsuba tells Kou that through this power of being No.3 he can see the life and death of his former self. He parallels his past, sad and lonely moments with his happy memories, ultimately exclaiming that he (the Mitsuba of the fake world) is envious of him (Mitsuba as a human). Shrouded by hope that he can live a human life in a fake world, he offers his hand to Kou (like he did in middle school as a human) asking him to become friends again. But Kou denies the offer exclaiming that all is pointless as the world and its contents are fake. Kou attempts to forcefully take Mitsuba out of the fake world, but is defeated by Mitsuba who is noted to have gotten stronger since their encounter in the Hell of Mirrors boundary. But Mitsuba is caught off guard when Kou once again tries to bring Mitsuba to some sense in regard to his situation and thoughts. It is revealed here that Mitsuba's self awareness is stronger than Kou had initially thought, as he begs Kou to tell him to his face that it is impossible for him to become a human again and to justify that the fake world situation is the best chance he has to fulfil his wish. Mitsuba tells Kou that he wishes everyone was dead so they could understand what he is experiencing and in saying this, Kou suggests he dies so they "can stay together forever." Kou subsequently jumps off the balcony of the school and Mitsuba jumps after him, using his bone exoskeleton to slow their descent as they both fall into a cluster of bushes.

After the incident, Mitsuba, presumably moved by Kou's actions, changes his mind saying that he will not help nor hinder Kou's attempt to get them both out of the fake world.

Picture of a Wish
Near the conclusion of the chapter, Kou sits in contemplation with Mitsuba evaluating Hanako's actions of trapping Nene in Mei's fake world. This is interrupted by Mitsuba and the two share a quick banter. The tone shifts when Mitsuba comments that Kou's actions need to align more with his words. ("You've gotta be able to follow through!") In hearing this, Kou decides that they should defeat No. 4 and while Mitsuba objectifies him, he is hit on the head with a paintbrush mid sentence.

Perfect Nonsense
Mitsuba is kneeling next to a sleeping Nene as he tell her to wake up. Mitsuba and Kou proudly show Nene that they have caught and tied No.4 up, although No.4 easily escapes from these ties.