Itsuki Akasawa has been Amane Fujimiya's best friend since preschool, granting him deep familiarity with Amane's personality and habits. He maintains a fiercely loyal and supportive friendship, frequently teasing Amane in a lighthearted way while also offering encouragement, especially concerning Amane's relationship with Mahiru Shiina. Itsuki possesses an honest and straightforward personality, though his impulsive actions sometimes land him in trouble. He shares a committed romantic relationship with Chitose Shirakawa, spanning several years; their mutual devotion is evident through frequent public displays of affection, often witnessed by Amane.

Initially, Itsuki held a negative view of Mahiru Shiina, perceiving her as a "spoilt princess" and advising Amane to avoid her. This perspective shifted dramatically after he and Chitose visited Amane during Christmas and discovered Mahiru was Amane's neighbor. Learning about Mahiru's discomfort with special treatment at school and her desire for normal interactions led Itsuki to actively encourage a romantic relationship between Amane and Mahiru, recognizing their mutual feelings. Together with Chitose, he directly facilitated their confession and relationship by creating opportunities for them to confront their feelings.

Itsuki also shares a friendly rapport with Yuuta Kadowaki, another classmate who befriended Amane. He expresses satisfaction that Yuuta also recognizes Amane's positive qualities, indicating he is not Amane's sole advocate. During a school event where Amane and Mahiru's relationship was unexpectedly revealed, provoking jealous classmates, Itsuki remained confident in Amane's ability to handle the confrontation independently. He later reacted with amusement when Mahiru intervened decisively to resolve the conflict.

Itsuki has a father mentioned in the narrative. Physically, he has brown hair and brown eyes. His birthday is March 4.