Yui Sendo

Kako and Kota met Yui Sendo for the first time during their summer vacation trip, which got almost completely ruined by a group of second-year students from Oto High who happened to be staying at the same beach resort. These students include Yui, Jiro and Mikado. The latter of the three knew of Kako and Kota's getaway for the summer but wasn't aware of where they were going.

During this trip, Yui discovers Kota and Kako's relationship by accident, however, she doesn't see why it would in any way shape or form affect her, so she doesn't tell anyone about it. In fact, she wasn't even sure of what she'd seen, only leaving her mildly skeptical of the two's relationship. Her suspicions are later confirmed in the story when she sees the two embrace shortly near a bus stop.

Yui and Heisuke consider each other to be siblings, despite being totally unrelated to one another and their official step-siblinghood only having lasted for a few years.

Just like her brother, she too comes from a broken home and needs the help of the police in order to get out of the horrid situation, too difficult for a teenager to solve on her own. This is where her being considered a friend by Kako helps her immensely.

Appearance
Yui is a very pretty girl, standing at about the same height as Mikado.

Her hairstyle changes drastically throughout the series; as a young child her hair used to be a little short of shoulder-length, in her introduction she's depicted with having hair that reaches her shoulders just barely, later on when she returns to the story, now in a larger role, her hair has grown out, and once her police case gets solved, her hair gets chopped to a pixie cut, after she uses her hair to start a fire.

Personality
Yui is very similar to Heisuke in her personality. She too pretends to be cold and unmovable, hiding her kind and respectful true self deep within a plethora of insulting words and icy glares directed at those around her.

Heisuke Okami
Yui and Heisuke share a close relationship.

Kako Motoya
Yui and Kako didn't get along at first but later warmed up to each other.

Natsuhiko Sendo
Yui feels a deep resentment toward her father, while still holding onto the thought that he could change his ways and actually become a good parent to her.

Mikado Yaguchi
Yui and Mikado are friends.

Jiro Nagakura
Yui seems to have a closer relationship with Jiro than, say, Mikado, seeing as Heisuke and Jiro are very close friends.

Ichika Nagakura
Yui is very caring toward Ichika, taking a big sisterly role in their interactions.

Kota Sagano
Yui doesn't seem to at first think much of Kota, other than that he's a meddling old guy who should just mind his own business. While she seems to keep up a front of thinking as much later on as well, in actuality, she might have warmed up to him at least slightly, especially after Kota saved her from a burning building.