Family
In the Volume 3 drama C.D it was mentioned that for his birthday, his family would prepare a cake and there would be a present under his pillow the next morning. Nothing else is known about them.
Soubu High School
Saika is popular among his peers. He has his own fan club comprised mainly of girls, who address him as prince for his feminine personality. He is quite popular with both genders and mentioned to be on par with Hayato in popularity.
Tennis club
Saika seems to have a good relationship with his club-mates. He is shown to be a capable club president and able to manage his members how he wants. Demonstrated in Season 2 Episode 11, where he uses the tennis club to block all other runners from catching up to Hachiman and Hayato. Saika is dedicated to improving the strength of his club-mates.
Hachiman Hikigaya
He was in the same class as Hachiman for two years and they attend the same gym session/class at school. Saika is very kind and caring towards Hachiman, often picking up on his bad moods. Saika tries to cheer Hachiman up many times throughout the series. Hachiman was Saika's tennis partner once, when Saika's usual partner was absent. Saika usually calls Hachiman by his first name. Hachiman sometimes responds in kind, even though it makes him uncomfortable, but Saika enjoys it
Hachiman was completely unaware of Saika's existence, despite being in the same class for two years. Hachiman attributes this to the fact he usually ignores girls in his class or everyone else in the class. In their second year of high school, Saika is introduced to Hachiman who finally learns that he is actually a boy.
Hachiman often has 'romantic thoughts' about Saika, and quickly stops himself from 'falling for the Totsuka route' after reminding himself that Saika is a male. Saika is one of the few people who truly wants to be closer friends with Hachiman, despite Hachiman's tendency to push others away. Furthermore, at the summer camp Saika is shown to sometimes dream about Hachiman, mumbling Hachiman's name in his sleep.
Saika is one of the few to know the real reason of the roof incident during the Cultural Festival, though he believes Hachiman has his reasons for acting the way he does and ultimately wants a good outcome. After the athletic festival, when their entire class shows their dislike for Hachiman, he still socializes with him. He admires Hachiman for his true, kind personality.
In Episode 7 of season 2 and Volume 9, Saika says that he thinks Hachiman is a cool person, indicating that Saika thinks highly of Hachiman.Saika also participated in a meeting to contributing in helping Hachiman to stop Yui & Yukino from participation as a candidate for student council president.Saika also showed interest and willingly teamed up with Hachiman a few times. In the light novel, it is also shown that he is able to tell when Hachiman is depressed or restless in many cases .They often seem to hang out together in places like the arcade and even to the movies during summer vacation (Volume 3, volume 5)
Hayato Hayama
Hayato and Saika participating in Hina's play.
Hayato and Saika seem to be on friendly terms. Saika and Hayato participated as the lead roles in Hina's play, an adaptation of the story "The Little Prince", though Hachiman was originally supposed to be in Saika's place.
Once Hayato tried to assist the Service Club with Taishi's request, but this ended poorly for Hayama. Hachiman along with Yoshiteru burst out laughing, Saika tried to stop them from laughing at Hayama but struggled to hold his own laughter. Hayato notices but has no hard feelings for Saika.
Hayato, Hachiman and Saika attended the workplace visit as a group. They seem to get along without any hardships.
Yoshiteru Zaimokuza
Saika occasionally hangs out with Zaimokusa. In Volume 3, Zaimokuza accompanied Hachiman and Saika to the arcade and to the theater. Saika gave Zaimokusa a picture of the three. Saika also extended his hand in friendship stating that "a friend of Hachiman's is his friend as well". Zaimokuza initially thought Saika was a girl at first until learning otherwise. Saika is not put off by Zaimokuza's tendencies and gets along fine with becoming the latter's first friend.
Service Club
Saika seems to hang out comfortably with the Service Club. He often helps the club in his spare time with their requests.
Yukino Yukinoshita
They have mutual respects towards one another. Yukino was impressed by Saika's determination, during his training.
Yui Yuigahama
Saika and Yui are classmates, and they are on quite good terms with one another. Yui is very fond of him and has thoughts of cross-dressing Saika and taking pictures of him. Yui was the one to suggest Saika ask the service club to help him improve in tennis. She also helped him train as part of the Service Club.
Yui invited Saika to a study group with Yukino. In the Volume 3 drama CD, Saika gave Yui a hair clip for her birthday. Yui affectionately calls him Sai-chan.
Others
Komachi Hikigaya
They seem to be on good terms. Just like her brother, Komachi, she too mistakes Saika to be a girl at first but was surprised to learn his real gender. She considers Saika to be very cute. Komachi sometimes worries about her brother paying lots of attention to Saika in a romantic way.
They were shown playing together in the river along with others at the Chiba Village Summer Camp. They call each other by their first names.