Jinta "Jintan" Yadomi is the main male protagonist of the story. Physically, he is a fifteen to sixteen-year-old boy of somewhat short height with a slim build. He has black, mid-length hair with bangs that fall across the middle of his face, and yellowish-brown eyes. His typical outfit consists of a red shirt with the Japanese characters for "underground people" printed vertically on it, which reflects his reclusive lifestyle, along with blue jeans and black and white running shoes. He also occasionally wears black-rimmed glasses. In his childhood, he had shorter, messier hair and was often seen in a watermelon-pink shirt with a yellow emblem. He is left-handed, as shown by which hand he uses to write and eat with a spoon.

Jinta’s personality undergoes a dramatic transformation following two traumatic losses. As a child, he was the energetic, outgoing, and natural leader of his friend group, known as the Super Peace Busters. He was athletic and seen as the one with the most potential. After the accidental death of his close friend Menma and the subsequent death of his mother, however, Jinta becomes withdrawn and isolated. He develops into a hikikomori, refusing to attend school and confining himself to his home. He becomes apathetic, socially awkward, and struggles to communicate properly with people outside his old circle of friends. He experiences symptoms of depression, including difficulty focusing his thoughts and persistent feelings of guilt and a loss of self-confidence.

Jinta’s primary motivation throughout the series is driven by the sudden reappearance of Meiko "Menma" Honma, his childhood friend who had died years earlier. He initially refuses to accept that the ghost he sees is real, believing her to be a manifestation of his own stress, which he calls "the beast of summer". Despite his skepticism, he is determined to help grant Menma's wish so she can move on to the afterlife, even though he knows this will cause her to disappear from him again. This mission serves as the catalyst that forces him to venture back outside and reconnect with his estranged former friends, including Naruko "Anaru" Anjou, Atsumu "Yukiatsu" Matsuyuki, Chiriko "Tsuruko" Tsurumi, and Tetsudou "Poppo" Hisakawa.

In the group's history, Jinta served as the de facto leader of the Super Peace Busters, a role he struggles to reconcile with in the present. His most significant relationship is with Menma, for whom he has harbored romantic feelings since childhood. He refuses to admit to these feelings when asked as a child, a moment that indirectly leads to the argument and subsequent events that result in Menma's fatal accident. This unexpressed love and his resulting guilt are central to his trauma. He also sustains a close, complicated friendship with Naruko Anjou. Anaru harbors romantic feelings for him and frequently visits him at his home, attempting to coax him out of his shell. Their dynamic is tense and awkward, but he demonstrates his care for her by defending her from her classmates' slander. His relationships with the other male members of the group, Yukiatsu and Poppo, are strained by the shared trauma and guilt over Menma's death, but Menma's appearance slowly forces them to confront these buried emotions.

Throughout the story, Jinta develops from a guilt-ridden, isolated shut-in into someone who can actively face his past and his friends. He is plagued by regret for not apologizing to Menma on the day she died, once thinking, "I thought I could just apologize tomorrow. But that tomorrow never came". His character arc involves learning to accept his own feelings, including his love for Menma, and finally being able to mourn not only her death but also the loss of his mother. This culminates in him leading his friends one last time to help Menma find peace. He has no superhuman or notable physical abilities beyond the unique circumstance of being the only person who can see and communicate with Menma’s spirit.