|Warning: information below contains some spoilers!|
|“||How can you be so smart and yet not understand something simple? I never wanted to see something like that||”|
—Yui Yuigahama, My Teen Romantic Comedy SNAFU, TOO!
Yui's appearance varies from book to book. When first introduced, she is described to have loose, shoulder length, chestnut-brown hair that would sway when she walked. She is also mentioned to not follow the dress code, wearing her skirt short, the top three buttons of her shirt undone, a heart charm on a gleaming necklace, and bleached tawny-brown hair. Her red "second year" ribbon is loose around her collar and hangs over her "pretty big" chest.
Later, her hair is shown to be usually worn in a loose side bun.
In the anime, she has fair skin, light orange hair that is almost always seen with the loose side bun and peach-reddish eyes. Yui is mostly seen wearing the standard school uniform; a black blazer with a white collared shirt underneath, a red ribbon tie, and plaid school skirt. She also wears black knee high socks. However she doesn't wear the uniform up to school standards. She wears her tie loose, and sometimes doesn't wear the tie or blazer at all. Her blouse underneath often has the top buttons undone.
During the fireworks festival, she wears a pale-pink yukata with a flower pattern and tied with a large crimson sash as well as wooden geta sandals.
Yui is a cheerful and outgoing girl; even when she's sad or upset she always has a smile on her face. She has said that she always wants to fit in, and that disagreeing with friends used to bring her anxiety. She can be considered as a typical high school girl who is curious about romance and squirms at the slight idea of it. Hachiman describes her as the typical good looking, dumb, popular type.
She has a friendly attitude to mostly everyone around her and usually ends up befriending them. She is not all deterred by those who refuse her kindness such as Hachiman Hikigaya and Yukino Yukinoshita, even after both criticized her. She has nicknamed the two, "Hikki" and "Yukinon" (respectively), which both dislike at first but eventually accept as normal. She has nicknames for almost everyone and even asks for people to give her her own nickname during her birthday party.
She is an expert at reading the mood and adjusting herself to it. Because of this she often hides her true opinion and states what others want to hear in order for things to go smoothly. Yui's very sensitive to the politics of the class. She's known as the "glue" of Hayato's clique, as she is always the one providing the energy and friendliness. She dislikes being a disturbance.
She admires both Yukino and Hachiman for being able to say whatever they think and the brutal honesty of their opinions. Yui has stated their honesty, straight forwardness, and independence are the reasons she joined the Service Club. After her experience with the Service Club, she was able to speak her mind a bit more, showing how much she developed socially.
Due to Yui's reputation and beauty, she usually gets hit on by many male students, which is clearly shown to the audience in Volume 6.5, but Yui gets away by using Yumiko as an escape route. Though Yui is both cute and social, she is mentioned having no boyfriend as most boys believe that they can't land a date with her and choose to drop out on hitting on her, as stated by Tobe.
In Volume 1, she accidentally admits to being a virgin, but is embarrassed because she thinks it makes her "less of a woman" and lame to still be one in their second year of high school.
She can be very determined when she sets her mind to something. Demonstrated when she decides to run for student council, as well as when she forces the events of Volume 11 to happen.
In her guidance counseling survey, Yui wrote she wants to get along with everyone and wished to be friends with everyone even after graduating.
Though Yui never stands out in academic, intellectual, and athletic endeavors, other characters always comment on her social intelligence and awareness; always being able to tell the mood of the whole group and her ability to make everyone comfortable. Although some people say she is dumb, Yui reminds people that she passed the high academic standards to enter Sōbu.She has the ability to sing well and enjoys karaoke.
She is a good coordinator and worker, able to follow and give instructions when needed (Athletic/Culture Festivals).
Though she is not very good at cooking, she tries her best and is committed to improving.
- (Greeting) - "Yahallo!"
- (To Hachiman Hikigaya) - "How can you be so smart and yet not understand something simple? I never wanted to see something like that."
- (To Hachiman and Yukino Yukinoshita about her indirect promise to win Hachiman's affections) - "If I win, I'll take everything. It's probably unfair. But it's the only thing I can think of. I want things to always remain this way."
- (To Herself after seeing the picture of Hachiman and Yukino) - "I actually realized it a long time ago. There was a place that I couldn't get into, and though I stand in front of that door time and time again, I get the feeling that I shouldn't disturb it. All I can do is peek in through a crack and eavesdrop... Actually, I realized it a long time ago. I knew I wanted to go there. I wanted nothing more than that. That's why, honestly... I never wanted anything genuine."
- "I think I was always charmed by it. That pretty smile she had was something I loved. And when I saw it then... I realized that this relationship was over."
- (To Hachiman) "I want it all. That's why i want Yukinon to be part of these afternoons filled with nothing. Wherever you and Yukinon are, I want to be there, too. So you have to tell her." 
- The name Yui means "tie, bind" (結) (yu) and "clothing, garment" (衣) (i).
- Yui's surname Yuigahama means "reason, case, cause" (由) (yu), "comparison" (比) (i) and "beach, seashore" (浜) (hama).
- Yui's first name is also in her family name (Yuigahama).
- Yui accidentally reveals herself to be a virgin and feels it to be embarrassing, since to her most people in high school are non-virgins.
- Yui has a dog named Sablé who was the one that Hachiman Hikigaya saved.
- Yui's email profile picture is also her dog
- Yui's pink phone has a skull and bones pattern, which is the same one as the first sticker on the volunteers club door sign (since she was the first student the club helped). Because of that, she is probably the one who puts the stickers on the volunteers club blank room sign (since the room is actually a storage room.) after each successful student helped.
- In Episode 11, Yui's ringtone is revealed to be "Bitter Bitter Sweet".
- Yui drinks tea in the volunteers club in a cup with a lethargic dog drawing.
- Yui received presents from Yukino and Hachiman for her birthday.
- Yui is one of the few to have Hachiman's mail address and phone number.
- Speculations are saying that Yui is behind the stickers she's putting on the Service Club's sign.
- For more info, see The Yui Theory.