Mitsuo Kubo first appears early on in Persona 4 as a student from another high school that attempts to ask Yukiko Amagi out on a date. However, his lack of social graces and generally off-putting demeanor cause Yukiko to reject him. His hopes dashed, he leaves without ever giving his name. He is later seen frequently in the Shopping District on the same days that Taro Namatame appears, ranting about everyday life to people who are willing to listen.
Mitsuo is an empty individual with no attributes, friends, or accomplishments aside from his love for video games and the Internet. He is ridiculed by many people due to his zombie-like appearance. Mitsuo is unable to accept his emptiness, and is unable to face society as a result. He makes up for his inability to live in society by hiding behind video games and the Internet. Due to Mitsuo's emptiness and lack of notable life, he also has an extreme desire for attention, whether it be negative or positive, to prove that he is worth something. As such, he attempts to ask Yukiko Amagi out on a date, as Yukiko is the most popular and beloved girl in Inaba whom all boys wish to date, which would make Mitsuo of high social standing as well. After he is rejected by her, Mitsuo attempts to take responsibility for the serial murder case in Inaba, boasting on the Internet that he is the killer. After he isn't taken seriously, he attempts to perform murder, killing Kinshiro Morooka in a lust for attention and power. After this, he believes himself to be a hero due to killing someone who was disliked by everyone. He becomes obsessed with murder as he feels that is the only way he can prove that he exists, let alone make any impact on the world. Mitsuo is also highly delusional, believing that he and Yukiko love one another, and that he is a hero due to killing Morooka.
Connection to the Murder Investigation
Mitsuo is a former Yasogami High School student who had been in Kinshiro Morooka's homeroom class the previous year. Morooka used Mitsuo as an emotional punching bag, constantly putting him down and exacerbating his already fragile self-esteem. There came a point where Morooka had Mitsuo expelled, but the details as to specifically why Morooka expelled him remain unexplained.
Deeply hurt, Mitsuo sees Morooka as his enemy, and inspired by the Inaba serial killings, he decides to play copycat. On the foggy night that Rise Kujikawa would have died had the investigation team not rescued her, he murders Morooka by clubbing him over the head and leaves him dangling upside-down from a window. Seeking attention for his "heroic" act, he then went to the police to admit to the crime and to take credit for the other serial murders, as well. The police don't believe him at first, but Adachi, sensing a way to continue his little "game," leads Mitsuo into an interrogation room before shutting off the lights and shoving him into the TV.
Mitsuo's Dungeon and Shadow
In the TV World, Mitsuo's heart crafts Void Quest, a dungeon themed after old 8-bit era RPGs, referencing the idea that he is a hero in his own mind. His Shadow, by contrast, is an empty shell, reflecting the fact that deep down inside, Mitsuo has no self-esteem, and hates himself with a passion. When Mitsuo rejects it and it goes berserk, it takes on a two-fold appearance. "Shadow Mitsuo," a fetus encased in the shell of "Mitsuo the Hero," a giant RPG character made of blocky pixels, similar to the art style seen in 3D Dot Game Heroes.
Even after the Shadow's defeat, Mitsuo continues to reject it. Yet rather than grow in strength again, the Shadow, still nothing more than an empty shell, vanishes, as Mitsuo's ego was corrupted, causing Mitsuo to become the truly malevolent being, in essence turning into his Shadow. After being pulled out of the TV World, Mitsuo is turned over to the police. Upon his arrest, he's charged with the murders of Mayumi Yamano and Saki Konishi as well as Morooka's, leading the police to consider the matter closed. Later, after Naoto Shirogane's rescue, it is revealed that Mitsuo was a mere copycat killer who was only responsible for Morooka's death. After Nanako Dojima's kidnapping, the police ultimately admit that they had made false charges. What became of Mitsuo after his arrest is unknown.