Pine is the childhood identity of the main protagonist of Punch Line. In his early years, Pine was a boy held at a research facility together with two other children, Chiyoko and Guriko, where experiments related to a process called Uberfying were conducted on them. During their escape from the facility, the three were involved in a car accident that caused their souls to swap into one another's bodies. As a result, Pine came to inhabit Chiyoko's female body, while his original male body came to be inhabited by Guriko's soul. After the accident, the person living in Chiyoko's body was taken in by the Iridatsu family, given the legal name Yu Iridatsu, and became known as Yuuta Iridatsu, the protagonist of the series. His older sister, Akina, is not a blood relative, and the existence of the body swap is one of the central secrets of the story.

In terms of personality, Pine's character is outwardly cool, but beneath that surface he is strongly justice-minded, compassionate, and sincere in a straightforward way. A notable quirk is that the sight of a girl's underwear causes him to get a nosebleed and lose consciousness, and this tendency plays a central part in the rules governing his abilities. His main motivations are to recover his original body, protect the people around him, and prevent the catastrophic chain of events that his own powers could set off, including the destruction of humanity.

Within the story, Pine is the true identity behind the protagonist, and the mystery of his past drives much of the plot. Over the course of the series, the protagonist gradually uncovers the truth about his origins at the facility, the nature of the accident, and the fates of his two childhood companions. One of them, Chiyoko, grew up as the cheerful idol Mikatan, who also operates as the heroic figure Strange Juice, and the two share a close bond rooted in their shared past. The other, Guriko, came to inhabit the body of the antagonist and ultimately stands against the protagonist in the final confrontation. The spirit cat Chiranosuke also serves as a persistent guide to the protagonist in his disembodied state, offering advice and setting the rules he must follow.

The character undergoes significant development through these revelations. Initially shown as a disoriented spirit trying to find a way back into his body, he gradually comes to terms with the loss of his original self and the complicated history he shares with his friends. In the final stages of the anime, when the possibility of restoring everyone to their original bodies arises, he chooses to refuse it, giving the body he has been living in back to his friend and accepting his own death so that the cycle of destruction can be broken.

As a former test subject, Pine possesses special abilities connected to the Uber experiments. His power is dramatically amplified by excitement, and while his soul is separated from his body, the rules state that if he becomes excited a second time in that state, the surge of power will cause an asteroid to fall and end the world. In his disembodied form, he is able to move unseen through the apartment building where the story takes place, and at a key point in the narrative, he also travels back in time to a past event in order to prevent a death and change the course of history.