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 actually the one from the real world, having been brought to this world by Mei in order to grant his wish to live as a human. He reveals that he has seen everything that the old Mitsuba experienced, both in life and death, and is jealous of him. He believes that Kou is not actually his friend, and does not want to be. Kou disagrees, and says that he still wants to be his friend, going so far as to say the two could die together in order to prove it.