First Manga Appearance: Chapter 01
Name Meaning: Mura=inconsistency; irregularity, Koshi=hips; waist, Ayumi=stroll; advance; progress
Occupation: Teacher at Tsujido High
Special Skills: Can speak 3 foreign languages (English, French and Chinese); can ride a moped for up to 2m; can do the crawl (swimming) for up to 3m; can ride shotgun but can also drive
Notable Features: Lost her virginity at 19; wears glasses
Ayumi fell in love with her 1st boyfriend & loses her at 19. They loved each other very much & her boyfriend's younger brother, Shindoji Fumiya saw her as an older sister. Sometime later, her boyfriend passed away on Xmas Day in a bike accident, causing her extreme grief. Fumiya reminds her that he will kill her if she ever loved another man. Unable to move on, she lives day by day. She eventually becomes a teacher at Tsujido High.
She goes on a holiday with her friend & colleague, Izumo Mariko during the summer holidays to the beach resort at Yoron Island. They are harassed by guys from Chiba & are saved by 2 waiters, Onizuka Eikichi & Danma Ryuji, who claim to be medical students, while Mariko & Ayumi pretend to be 2nd year college students. Although initially indifferent, Mariko manages to convince her to forget about her past & have fun. She starts to take an interest in Ryuji while Mariko has her eyes on Eikichi. She almost sleeps with Ryuji but is interrupted by his bad stomach. Due to a fight at the disco, she discovers that Eikichi & Ryuji are just high school delinquents.
On the 1st day of school, she is shocked to find that Eikichi & Ryuji are new transfer students at Tsujido & begins to plot her revenge with Mariko against them. When the Oni-baku blackmail her & Mariko to have sex with them or else they reveal the Yoron Island incident, Mariko lures them in & calls her father's homosexual Yakuza members to take their place. Upon discovering Mariko's plan, she asks Mariko to call it off & rushes towards her room, to see Ryuji leaving, meaning that he didn't go into the room. After discovering that they have things in common, the two spend the night in a different hotel room but Ryuji leaves when she reveals to him about her ex-boyfriend.
She goes on the Xmas ski trip after Mariko reveals to her that Ryuji & Eikichi also plan on going. She bumps into Ryuji during Xmas shopping before the ski trip & quickly heads off upon seeing Shirayuri Momoko hug Ryuji. An embarrassed Ayumi drops her ex-boyfriend's sweater & Ryuji mistakes it for his Xmas present. On the trip, she witnesses Momoko confess her love to Ryuji, declaring that she won't let Ayumi have Ryuji. However, Ryuji chooses Ayumi over Momoko & meets up with her to & they exchange Xmas presents. Ayumi explains to Ryuji that the sweater earlier belonged to her ex-boyfriend & that today was his death anniversary.
She takes 2 months off from her job & meets Ryuji at a disco upon her return. She asks Ryuji out on a date. She visits her ex-boyfriend's grave one last time & bids him farewell before the date & is watched on by Ryuji, who thinks that she is still not over her previous lover. He confronts her on their date at the amusement park & she shows him the location where her ex-boyfriend died, explaining to Ryuji that she was bidding her last farewell.
Upon discovering Ayumi's decision to follow an arranged marriage her parents had set up, Fumiya returns & kidnaps her for being unfaithful to his brother. Ryuji goes after Fumiya & Ayumi gets injured in the process, resulting in her hospitalisation. She reveals to Ryuji the truth about her marriage & her plans to quit teaching. However she can't forget about her ex-boyfriend & made her decision to marry because she doesn't want to be alone. She thanks Ryuji for everything & leaves to protect Ryuji from Fumiya.
Despite leaving Ryuji, Fumiya still goes after the Oni-baku. Fearing for her students, she rushes to Hayama where Eikichi was beaten by Fumiya & gets kidnapped by Fumiya once more. Fumiya uses her to lure out Ryuji. Fumiya is eventually beaten by Ryuji & realises that he was childishly trying to look for his brother in Ayumi. Ryuji saves her & reveals his true feelings. Realising that she loves Ryuji more than anything, she confesses her love for Ryuji. Later, Fumiya apologises to Ayumi for what he's done before he is taken away by the police.
Ryuji asks Ayumi to marry him & they make plans to meet up with Ayumi's parents. However, Ryuji arrives to the meeting late due to having to help Eikichi fight Okubo Mitsuaki. Despite her parent's disapproval, she tells Ryuji that she loves him because of his free life. They head to a hotel room to connect but Ayumi stops herself from having sex with Ryuji. She changes into a wedding dress & the two head off to an empty church.
Ultimately, she decides that as an adult & as Ryuji's teacher, she cannot bear to ruin his life by denying him of his youth & taking him away from his friends. She returns to Niigata to patch things up with her parent & leaves Ryuji, leaving her feelings for him in the form of a letter.