Yui lived and grew up in a big apartment complex with lots of children of her age. She always tried her best to get along with them. When all of the children played house, in order to make everyone else happy, she ended up becoming the dog even though she wanted to play mother because another girl wanted to play mother.
Yui stated that she didn't have any romantic relationships in the past, but her friends did. She is only in contact with one or two friends from elementary school, even though she used to be friends with all her old classmates.
During the Chiba Village summer camp, Yui was hinted to have a guilty conscience. Yui might have wanted to befriend some ostracized person but she was afraid of the "consequences" that might happen and hence avoided such possibility. In the light novel Hachiman thinks that in the past, Yui might have done something that had worsened the relationship between herself and the rest of the class.
On the first day of high school, before the high school entrance ceremony, Yui went for a jog with her dog, Sable. The dog got off its leash and ran into the road, right in front of a speeding car. Fortunately, Hachiman Hikigaya saved her dog from a potential car crash by leaping in front of the car, leaving him injured and hospitalized.
As Hachiman recovered, Yui visited his home and gave his sister sweets as a thank you gift for protecting her dog, but Komachi ate all the sweets herself and never told Hachiman about Yui's visit. It was not until their second year, where Komachi finally told him about the owner of the dog and that she was in the same school as him but she had forgot Yui's name.
Once Hachiman returned to school, he didn't seem to recognize Yui from the incident. Yui didn't have the courage to speak up about it. Its unknown whether she kept tabs on him or tried to gain strength to approach him.
In their second year, Yui wanted to make homemade cookies for someone and requested help from the Service Club. While there she immediately recognized Hachiman, who did not recognize her, but was annoyed by the nickname "Hikky" she gave him.
As the club conducted her request, Yui was amazed by the fact that the two can speak their mind and had no problem putting people down when they criticize her cooking and effort. The Service Club had a hard time completing her request because Yui was a terrible cook and wanted to give up quickly. Eventually the request was fulfilled and Yui rewarded the club with burnt cookies. She later asked to be friends with Yukino and says she'll join the club.
One day Yui is trying to leave her group of friends to have lunch with Yukino. Yumiko bullies her until Yukino intervenes.
At one point, Yui questions Hachiman about his past accident, but doesn't reveal that she was the owner of the dog he saved.
Yui is the one that brings Totsuka to the Service Club with his request. During Totsuka's tennis match Yui initially partners with Hachiman until she hurts her leg. She gets Yukino to replace her in the match.
Yui is looking for Hachiman for Club one day and finds him with Shizuka. She asks for his phone number so she wont have to go looking for him next time.
After this she keeps receiving mean texts about people in her class, Yukino assumes its Hachiman. It is not.
After school Yui and Yukino run into Hachiman who is studying for midterms. They decide to study together but get distracted when Komachi walks in. Yui accidentally lets Hachiman know she is aware of who Komachi is, before being introduced.
Hayama shows up one day with the same text messages that Yui is getting. He wants them to stop. Yui and Hachiman investigate their class to see if they can find anything. Yui has no luck in finding the culprit.
During another study session, Komachi brings Taishi to the Service Club for help. They accept his request and investigate Saki. One of the places they check is a maid cafe where Yui happily dons a maid outfit in order to roleplay and spy on the other workers. Later that night Yui accompanies Yukino and Hachiman into a fancier restaurant to find Saki.
After Taishi's request Komachi tells Hachiman that Yui is the owner of the dog he saved.As a result of this knowledge, Hachiman confronts Yui about this and expresses his belief that her kindness to him stems from guilt. Yui denies that her feelings come from guilt, and is brought to tears that he would think such a thing before running off.
It is revealed that Yui has stopped coming to the Service Club and is no longer texting Yukino. During her time away, Yui associates with her other friends.
One day, at the "Tokyo Cats and Dogs show", Yui is taking Sable to get groomed. Sable manages to get off her leash and runs into Hachiman, who happened to be there with Yukino. Yui catches up and was shocked to see the pair, assuming they are on a date. She becomes sad and tries to leave but Yukino invites Yui to a meeting which Yui believes is for them to explain that they are now a couple.
On Monday she attends the meeting ready for her assumption to be confirmed but they are interrupted by Zaimokuza with a request.
Afterwards they return to the Service Club, Yukino reveals the discussion was to celebrate Yui's birthday and welcome her back to the club. Yukino mentions they have gifts and Hachiman gives Yui the present he bought. Hachiman then tries to clear their relationship properly. Yui says she doesn't want their relationship to end. Yukino helps them get over their awkward situation and tells them to start fresh. She then leaves to tell Shizuka that they have recruited an additional member.
After Yukino leaves, Yui opens Hachimans gift. It's a dog collar she mistakenly believes is a fashionable piece of neck-wear. When Hachiman explains what it really is she becomes playfully upset and goes to leave. Before she does, she says "thanks, idiot" and mentions she will book a place for her party, then runs away. After this, she returns to the club with everything mended between them.
The day of her birthday party Yui runs into Shizuka and invites her along to karaoke, she refuses saying she has somewhere else to be. Yui also takes pity on Zaimokuza and invites him along too.
Yui has a fun party and receives gifts from everyone except Zaimokuza. She reveals her blood type is O. She sings several Duets with Yukino, one of which is "Bright Generation".
Yui meets Hachiman during summer break and makes him promise they will do something later.
Yui goes with the Service Club to the Chiba Village summer camp. While there she meets Rumi and wants to help her.
After the Chiba summer camp, a car came to pick up Yukino and Yui recognized it as the car that hit Hachiman. After the car drives off she and Hachiman acknowledge that it was the same car that hit him. They are left thinking Yukino also knows about the accident but hasn't said anything to either of them yet.
After the Summer Camp events Yui is going on vacation with her family. She shows up at Hachiman's house and leaves Sable with Komachi to dog-sit.
When Yui picks up Sable she offers to go to the Fireworks Festival with Hachiman. He accepts only because Komachi convinces him.
The night of the fireworks Hachiman goes to meet Yui. She shows up late and they awkwardly get on the nearest subway. On the way they chat awkwardly. Hachiman notices Yui is unusually shy, and very cute in her Yukata. Yui mentions she used to go with her friends to see the fireworks but this time is going with Hachiman.
They arrive early. Yui mentions Komachi sent her an email with a shopping list attached. When shopping Yui keeps getting distracted by the games stalls. Later they run into Yui's acquaintance, Sagami. Hachiman immediately notices Sagami is judging him and realizes he has hurt Yui's "social reputation" by being with her. He distances himself to lessen Yui's "embarrassment" and goes to get Yakisoba. When Yui catches up its almost time for the fireworks to start.
They look for a place to sit when they are noticed by Haruno Yukinoshita. Haruno invites them into her VIP section to watch the fireworks. Haruno thinks they are on a date and throws small insults towards Yui. Yui denies that its a date.
After the fireworks Haruno offers to give Hachiman and Yui a ride home, albeit in the same car that hit Hachiman when he started high school. Haruno acknowledges the collision saying all the scratches are gone. Hachiman and Yui say nothing and realize that Yukino really did know about the accident. Haruno realizes her mistake and tries to defend Yukino. Hachiman and Yui opt to walk home instead.
They were silent the short ride to Yui's station until Yui starts a conversation as she gets off. Hachiman offers to walk her home and talk about the accident. They agree to still pretend they don't know about the accident in order for Yukino to come forward on her own terms. Yui says she still wants to know Yukino's feelings about the accident, and learn even more about her in order to be better friends and help her. She then makes Hachiman swear to help Yukino if shes ever in trouble. Yui' is tearfully trying to explain something else about her feelings towards Hachiman when her phone rings. She answers it at the behest of Hachiman. Its her mother wanting her to hurry home, Hachiman watches her run down the street into an apartment building before going home himself.
At the time of the cultural festival, Yui became the assistant producer for the class play. In times of the first huddle she was slightly disturbed by the fact that Hachiman was not included or completely ignored by the class, Hachiman who was sitting at the reception alone, but Hachiman signaled her to be alright. She asked Hachiman if he was upset of not being included in the first huddle but Hachiman says other-wise much to her relief. Yui adds that this would be Hachiman's answer. Since Hachiman feels that he was serious about the weirdest things and not things which are sensitive to others.
She later volunteered as candidate for Student Council president after realizing Yukino's participation. Later she planned on choosing Liberal arts for her career path for third year
In the later series of Episode 11 (adapted from Volume 9 chapter 10), Yui has been shown to be very observant of the relationship of Hachiman and Yukino. Yui might also suspect they have feelings for one another. (In the Volume 10, chapter 7) Yui thus tries to observe Hachiman and Yukino chat while they're alone in infirmary, hoping to get some clues about their relationship.
When Hachiman opened the infirmary's door on the way back to his classroom after being treated by Yukino, he finds Yui standing in front of the infirmary, and asks her whether she is get there just now, and Yui replies that she just got there with a shocked expression. Judging from her expression, it is implied that Yui has been there for a while, and not to mention also hear Hachiman and Yukino chatting when they're alone in the infirmary. Hachiman also suspected that she was lying in her statement but didn't call attention to it.
In Episode 13 of season 2 (adaptation of Volume 11), Yui invited Yukino to stay at her place and the next morning during their free time she asked Hachiman for the promised date and invited Yukino along. They spent time together, and by the end of the date Yui mentioned about the battle royale and stated that she would take everything by it and also put forward her request for the service club status quo to be maintained by hiding their true feelings. Yukino almost agreed but Hachiman declined her request.
Addressed / Nick names
- YuiYui By Hachiman Hikigaya
- Ghama-chan By Haruno Yukinoshita