Iroha Igarashi is a high school student who serves as one of the central characters. She is known throughout the school for her striking appearance, which includes wavy, waist-length light pink hair and light brown eyes. Her fashion is typically flashy, and she wears the standard school uniform with a notable pink bow. Her reputation, however, is complicated. Many of her female classmates dislike her, and she is widely gossiped about as a promiscuous person, often labeled a sleazy girl who steals boyfriends. This reputation stems from her behavior of dating multiple boys and her blunt, straightforward manner of speaking, which some find off-putting.

Despite this harsh public perception, Iroha’s true personality is quite different from the rumors. She is fundamentally kind-hearted, honest, and easy-going. While she skips classes and appears to goof off, she also shows a strong sense of determination by diligently attending remedial classes and fulfilling her punishment duties. The rumors about her being sexually active are false, and she is, in fact, a virgin. Her actions are driven by a deep-seated loneliness and a desire for a genuine connection. She wants a boyfriend who will listen to her and appreciate her for who she is on the inside, rather than simply being attracted to her good looks. She has a playful side but is also shown to have a calm demeanor, which is partly attributed to her suffering from asthma.

Iroha’s role in the story begins when she and the otaku outcast Hikari Tsutsui are forced to clean the school pool together as a punishment. Despite Tsutsui’s own terrible reputation as a creepy otaku, Iroha is a good judge of character and sees the good in him, defending him from bullies when others are mean. She takes an interest in him and asks him out, and though he initially rejects her out of mistrust, he eventually reciprocates her feelings. Iroha accepts his confession, but with the condition that their relationship will only last six months, as she is scheduled to transfer schools. This sets the stage for a relationship that is meant to be temporary but becomes deeply meaningful.

Her key relationship is, of course, with Hikari Tsutsui, whom she affectionately nicknames Tsutsun. She is fiercely protective of him and their small circle of friends, including Yuto Ito and her first female friend, Arisa Ishino. Iroha is slow to forgive anyone who speaks rudely about the people she cares for and shows great compassion in trying to help them with their problems. While she is devoted to Tsutsui, his deep-seated insecurities can occasionally create difficulties in their relationship, but her love and faith in him never waver.

Iroha undergoes significant development and faces tremendous challenges. Her most notable ability is perhaps her resilience and her skill at seeing past people's facades to their true character. The narrative takes a dramatic turn when it is revealed that she has a brain tumor. After a necessary surgery, she suffers from retrograde amnesia and forgets her entire relationship with Tsutsui, moving to Los Angeles for treatment and to live with family. Years later, she returns to Japan, still with no memory of her past. However, when she sees a keychain that is part of a matching set she once shared with Tsutsui, her memories begin to return. This leads to a powerful reunion, and she ultimately chooses to move back to Japan to be with him. At the conclusion of the story, Iroha and Hikari get married, and she becomes pregnant a year later, signifying her complete journey from a lonely, misunderstood rebel to a person who has found love, belonging, and a family of her own.