Yui's appearance in Chapter 91
|Height||176 cm (5'9")|
|Hair Color||Light Brown|
|Occupation||2nd Year High School Student|
|School||Midori Ga Oka Academy|
|Affiliation||Student Council (former)|
|Family||Mother, Father, Cousin|
|First Appearance||Chapter 28|
|“||Enemies are enemies. Allies are allies. I wish things were simpler.||”|
Yui is a young man of average build, with unkempt brown hair and hazel eyes. It is noted that his hair is not permed, just unruly. Yui dons a pair of thin-rimmed glasses, though has been seen without them. He also once wore sunglasses similar to the members of the lower ranks of the Student Council.
He usually wears the Midori Ga Oka uniform- a white shirt and dark pants. During his time on the Student Council, Yui wore a dark blue shirt (custom made after asking Hanabusa Miyabi for a cool looking outfit). Yui owns many ninja outits, as well as lord wigs and kimonos.
Despite his appearance and often stoic expression, Yui is actually quite silly and goofy. He almost always has a plan (however unusual) to help a friend achieve a goal. Yui often overlooks things such as fine print on flares.
He has been described as a man of action, though lacking decisiveness, having given up thinking for himself after meeting his lord, Miyabi. Outside of times when Miyabi cannot be contacted, Yui will follow in with another person's decision, anyway (unless ninjas can be brought into the decision). He is quite oblivious, unknowing of Houjou Wakana's feelings towards him. Kurosaki Mafuyu has described him as being 'cold-blooded' at times, because of his social awkwardness.
Yui pretends to dislike unnecessary things such as friendship and worry, though actually truly enjoys having friends and partners, and in particular, is best friends with Hayasaka. He can be serious at times, with a stoic facade- mainly so when speaking to Miyabi
He has an obsession with ninjas and lords, being educated in all aspects of them, even owning real weapons and tools used by ninjas. Yui spent a portion of his time in junior high searching for the perfect lord to serve, who he later deemed to be Miyabi.
Yui enjoys gathering information, and only trusts data collected by himself. He will go to strange means to collect information, and often dresses up as a ninja (or other disguises) to do so.
Yui grew up as an only child, with his mother and father. One day, while watching shows, he became entranced by a drama about a ninja and his lord. His mother called him to ask which slice of cake he wanted between a strawberry cake and Mont Blanc.
After five minutes, Yui finally decided that he would choose the strawberry cake, and ate it while continuing watching the ninja drama. He then asked his mother how he could become a ninja.
Yui later attended a junior school, where he met Miyabi and became his personal ninja. Miyabi once asked Yui if he was close to anyone in his class, and he replied asking if Miyabi was ordering him to make friends. Miyabi asks if they are friends, but he furiously states that Miyabi was his lord.
Throughout the years, Yui followed Miyabi and his orders, even joining the same high school as him, Midori Ga Oka Academy, where he joined the Student Council to further serve Miyabi and his needs.
Miyabi is the president of the Student Council, as well as Miyabi's 'lord'. Yui has served Miyabi ever sincer junior high, and is completely loyal to him. Though Miyabi sometimes makes cruel jokes regarding Yui and his failures, Yui deeply enjoys his company and respects him. Even after quiting the Student Council, Yui still regularly visits the room to help Miyabi.
In the Art of the Echo arc, Miyabi orders Yui to betray the Public Morals Club as a test of his loyalty. Though Yui succeeds, then fails in the end, Miyabi forgives him. Haysaka tells Yui that all 'orders' being made by Miyabi are simply Yui being lazy and wanting an excuse to act like a ninja, which Yui later realizes with regret. However, Miyabi doesn't mind and assures Yui that he sees him as a friend, rather than a servant.
Mafuyu considers Yui to be a close friend. At first, Mafuyu doesn't trust him as he is a former student-council member and he tries to take the Disciplinary Club down. However, he loses against Usa-chan Man and joins the club as a result. His obsession with ninjas (and goal to become one) earns him the nickname "Ninja" by Mafuyu. She also appears to be quite jealous of his best-friend relationship with Hayasaka, and argues that she is his best friend because she had met Hayasaka three months before Yui had.
As of Chapter 94, he is aware of Mafuyu's past as banchou and of her personas Super Bun and Natsuo. He has hung out with Mafuyu's Saitama friends.
Though they got off badly at first, Yui and Hayasaka both have a strong friendship, and Yui even considers him his best friend. Unknowingly, Hayasaka first met Yui when Mafuyu tried to convince him that a ninja was lurking around. However, Hayasaka did not believe her, and continuously missed him while Mafuyu tried to point out the ninja. Hayasaka began punching walls where the ninja supposedly was, and even used plastic wrap to cover the air hole of a bamboo pole which Yui used to breath through underwater.
When they meet again (with Hayasaka seeing his face), Haysaka does not realize the damage he had done to his pride. However, over time, their relationship gets better, and Haysaka begins to consider Yui a great friend and fun to have around.
During Yui's betrayal, Hayasaka still trusts him, saying that the Public Morals Club did not need Kawauchi Tomohiro (an informant), when they already had Yui. Hayasaka is the one who catches Yui when he jumps out of the dorm windows.
In his Volume 17 Character Profiles secion, Yui says that based on their total time spent together (in the dorms), Hayasaka is his best friend. On the days that Mafuyu eats bento lunches with Ayabe Reito, Hayasaka eats lunch in the cafeteria with him.
- Yui is voiced by Tomokazu Sugita in the Drama CD, who is known for his roles as Kyon in The Melancholy of Haruhi Suzimaya and Gintoki in Gintama.
- He placed eighth in the official character poll, with 855 points. Izumi Tsubaki states that if the poll had been conducted any later, Yui would've lost most of his points because of his actions in Volume 16.
- His favorite food is natto on rice, but he hates bread and mainly eats in the school's cafeteria.
- His personality preference is someone who will always stay by his side and pay attention to him.
- He dislikes the internet, horror related things, thrill rides, and staying in one spot on his own.
- Yui is most close to Hayasaka than Kurosaki since they are both live together in the boys dorm.
- (To himself) "Everything I do is for Mr. Miyabi. I will do as my lord commands. I'd do anything for you..." 
- (To Hanabusa Miyabi) "Now then, Mr. Miyabi, give me your orders. 'Come back'... and 'betray the Public Morals Club to destroy them." 
- (To Kurosaki Mafuyu and Hayasaka) "I want to... be with you..."