Spoiler Alert! This article or section contains details not covered in the anime. Do not proceed unless you wish to be spoiled.
Student Council
Miyuki Shirogane
Miyuki is a very close friend of Ishigami. He was recruited by Miyuki after investigating a particular incident wherein Ishigami was framed to be the culprit. Miyuki finding out the truth and believing in Ishigami apparently "saved" Ishigami, thus a strong bond of trust ties them together. They even went to the aquarium together with only the two of them after Miyuki was turned down by Kaguya.
Kaguya Shinomiya
Kaguya is Ishigami's upperclassman. The two of them start of with an adversarial relationship, with Ishigami believing that Kaguya wanted to murder him, but eventually the relationship develops into a sibling-like bond (albeit still the occasional fear of her). His stance of pure fear seems to have permanently shifted around when he joined the cheer team and needed a female uniform, at which point Kaguya offered to help him. Eventually as exam season crept up again, Kaguya took note of his crush on Tsubame Koyasu and devoted herself to helping him pursue her, namely by helping him study so that he will rank high and Tsubame would see him in a better light. As the exams started, Ishigami reasons that he was able to try so hard at studying not because of his fear of Kaguya or his feelings for Tsubame but rather because "there is still someone that would believe in a guy like me" thinking of Kaguya. Ever since, their relationship has built progressively on this with Kaguya offering him advice, support for his love, and encouragement when he thinks on confessing. Ishigami now openly thanks Kaguya for her help.
Ishigami holds Kaguya's opinions to high regard. He regularly asks her for her opinion on ideas he has for romantic gestures towards Tsubame (many of which Kaguya finds creepy), and also asks to help him get better at volleyball to impress Tsubame. When he found out that Kaguya herself was in love with someone, he nonchalantly proclaims that he would stop her if he thought the person she was in love with may hurt her or wasn't worthy of her.
Miko Iino
Miko has been a schoolmate of Ishigami since middle school. Ishigami being a regular rule breaker and Miko being a member of the Discipline Committee led to their current relationship being like that of cats and dogs. They seem unable to get along. Nevertheless, their beliefs cause Ishigami and Iino care for each other without desiring recognition. Ishigami changed the Presidential Debate from speeches to a debate, which allowed Miko to overcome her stage fright. On the other hand, Iino had actually been one of the people who protested the punishment that was administered to him by his middle school, which eventually led to him being enrolled in Shuchi'in Academy. Additionally she was the first one try to move in to cheer him up when he failed to place first on the relay race.
After the Bonfire, Miko seems to have fallen for Ishigami after he gave her a heart-shaped pendant. Ishigami intended to turn over the pendant, found at the archery range, as a Lost-and-Found item. Ishigami still wasn't aware of the consequences of his action - a direct confession.
Following the accident after Tsubame's party, where Miko broke her arm whilst saving Ishigami from injuring himself, their relationship begins to change significantly. Out of guilt, Ishigami spends a lot of time making up to Miko by helping her write her homeworks and eat her food. After following Chika's advice to play nice with Miko, Ishigami comes to see a different, sweeter side of her.
Chika Fujiwara
Chika Fujiwara is a fellow member of the student council. The two tend to have a tense relationship as they constantly call each other out on their faults (her referring to him as creepy, and him catching her cheating at games). Ishigami is often the only one who can counter Chika's more chaotic aspects while Chika enjoys teasing him whenever she's given the opportunity to.
Later, it's revealed that the two hold some fondness for each other (Chika being sympathetic to his situation and Ishigami admitting her genius capabilities). The two are shown to care for each other as friends.
Friends
Tsubame Koyasu
Tsubame is the object of Ishigami's affection and a fellow member of the Cheer Club for the Sports Festival. After the Sports Festival, Ishigami is shown to have a crush on her, which is noticed by every other member of the Student Council.
During the Culture Festival, Ishigami won a giant heart cookie and gave it to her, which was taken by her as a confession due to the theme of the festival of giving a heart to someone you like. Though initially going to gently reject him, after speaking with Kaguya and watching Ishigami act in contrast to the rumors about him, she asked him if she could give him an answer after getting to know him more.
Ishigami got her to agree to a date to the Tokyo Skytree following his performance in the intramural volleyball tournament. Their date, according to Ishigami, went without a hitch. The two have scheduled their next date.
Maki Shijo
Maki is Ishigami's upperclassman. The two become friends due to her repeated visits to the student council room, and her relationship with him mirrors that of Kaguya's.
Rei Onodera
Rei is one of Ishigami's classmates and a fellow member of the Cheer Club for the sports festival. She is initially dismissive of him due to the rumors regarding him and Kyoko, but the two are shown to have become friends by the end of the Sports Festival.
Karen Kino
Karen and Ishigami have a professional relationship, where he acts as the editor for her Miyuki/Kaguya doujinshi. However, Karen does not consider the two of them to be friends.
Tsubasa Tanuma
Ishigami dislikes Tsubasa due to his 'Alpha' status and the fact that he used to generally disdain couples in the past. He doesn't hold him in very high regard, as he often wonders why Maki is so obsessed with him.
Kazeno
Kazeno was the captain Cheer Club, and like most of the members was shown to be fully accepting of him despite the rumors of his relationship with Kyoko. The two remain friends after the Sports Festival, and Kazeno is shown giving advice to him on how he can win Tsubame's heart.