Yaegashi Rina (八重樫 莉奈) is the younger sister of Taichi Yaegashi and the two are shown to be quite close. She typically holds the responsibility of waking her brother up in the morning and is very interested in Taichi's love life. Despite her young age, Rina will often give her older brother relationship advice. At the start of the series, Rina is in the 5th grade.
Rina is a young girl of average height for someone of her age. She has long, brown hair and brown eyes. Her bangs are split in a fashion that resembles her brother's.
Wise beyond her years, Rina is shown to be very fascinated with and knowledgeable about relationships and love. She is implied to be quite studious and knows very complex vocabulary for her age, which always surprises Taichi Yaegashi. Rina is also surprisingly knowledgeable of, and even likes to joke about, very "mature themes" that most kids her age probably should not know of.
Rina is shown to be a caring younger sister, diligently waking Taichi in the morning and expressing concern when he is feeling down. Rina can also be rather possessive of her brother, becoming jealous when he neglects her in favor of his friends at the Cultural Club. She is not very helpful around the house and heavily relies on Taichi to do most basic chores for her such as washing her dishes.
- Father: Rina's father is often away on business trips, trusting Rina to her mother and older brother's care.
- Mother: Rina's mother frequently dotes on her and dedicates much of her attention to Rina. She frequently reminds Rina to finish her food and not be a picky eater.
- Taichi Yaegashi: Rina’s older brother. The two share a very close relationship and are shown to care a lot about each other's personal lives. Taichi is sometimes worried why his sister seems to know more about love than he does. In addition, Taichi is very protective of Rina and seems to suffer anxiety any time he believes someone is interested in his younger sister. During the Nise Random arc, Rina claims to have a boyfriend who is three years older than her in middle school and a horrified Taichi demands she break up with him. Rina also heavily relies on her brother to do most chores for her, and Taichi teases that she will be a bad wife.
- Boyfriend: Rina's apparent boyfriend who is a third year in middle school, three years older than her. The two frequently talk over the phone and Rina is left bored if she is kept from speaking with him. Taichi is uncomfortable about his sister being in a relationship, and becomes very anxious anytime the two interact. Her boyfriend is always very busy with his club activities and, as a result, usually does not have time to go out with Rina on the weekends.
- Himeko Inaba: In Asu Random, Rina considers Inaba a rival, fearing that she will take Taichi away from her. The two do not get along and are prone to bickering with one another. The first time they met, they had a fight that Taichi does not like remembering. When Inaba visted, Rina interrogated Inaba about their relationship as if she was their mother-in-law. Then they had a debate on whose bond with Taichi is stronger but did not have a conclusive ending.
- Aoki Yoshifumi: Rina and Aoki have met a few times in the past when Aoki would come over to Taichi's house to hang out. She at one point believed Aoki and her brother to be in some sort of relationship.
- Yui Kiriyama: Rina briefly interacts with Yui when she and the rest of the StuCS visit Taichi's home. Upon sitting down with her, Rina is immediately smothered by Yui who exclaims how adorable she is. Ultimately, Rina's only thoughts about Yui is that she is scary.
- Maiko Fujishima: Rina and Fujishima meet for the first time in Yume Random when Taichi lets Fujishima use their house's bathroom. However, after Taichi witnesses Fujishima's desire of doing "inappropriate" things to Rina, he kicks Fujishima out of the house before they can interact any further.
- Nagase Reika: A friend of Rina's mother. Rina first became acquainted with Reika at Yamaboshi's Cultural Festival. In Asu Random, Rina and her mother coincidentally meet Reika again and they discuss the odd behavior of Taichi and Iori. Rina and Reika later meet again and decide to visit Yamaboshi High School together, fearing that their loved ones' strange behaviors are the result of bullying.
- Enjouji Shino: Rina did not like Shino from the beginning because she was worried that at school, Shino is a replacement little sister for Taichi. Shino even said that she wanted to become Taichi's little sister which infuriated Rina and the two ended up not getting along.