Kotonoha's family is very wealthy, with both her parents having extremely lucrative jobs. Her family lives in a large two-story residence surrounded by a wide and serene yard, also has several villas in different countries.
Since her parents are always busy Kotonoha and Kokoro are often left at home, and even if they are in the country they come home late at night, which provides many opportunites for characters to have sexual relations.
Kotonoha Katsura loves and respects her parents (Mr. Katsura and Manami Katsura) very much, and refers them with high honorifics. Her parents cares about her very much too, however they also put too many expectations on their daughter, for example to ask her to study and practice various skills since childhood. Her dad is very protective of her and is worried when he hears that she has a boyfriend and can take a day off work from his very busy schedule to meet him. However her mother is very different and helps her with her boyfriend and encourages her to pursue her love life.
Her parents are always busy and frequently go abroad on business, they always bring her and Kokoro expensive presents.
Although often get teased around and embarrassed by her outgoing and energetic younger sister Kokoro, she and Kotonoha care and worries about each other very much.
For instance Kokoro worries her elder sister can't enjoy life for being so shy and reserved, and feels very happy when acknowledges Kotonoha has Makoto as boyfriend. On contrary, Kotonoha worries her younger sister's safety for being so outgoing.
Makoto is Kotonoha's first and so far, only love. In all mediums of the series, she's asked out by Makoto which is reciprocated by her. She initially is very uncomfortable with the sexual demands from Makoto but if she gets over her psychological block her relationship with Makoto can be tightened greatly. Of course things can go differently and she can be abandoned by him, or be strung along and driven to the point of insanity.
In Summer Days, Kotonoha and Makoto have noticed each other and begun to develop feelings since they take the same train to school. If the player takes Kotonoha's route they become a happy couple during the Summer Carnival. If the player takes Kokoro's route, they can also become a happy couple, but Kokoro will be Makoto's another girlfriend.
In Cross Days, Kotonoha and Makoto's relationship have already been endangered by Sekai and others, but the protagonist Yuuki Ashikaga volunteers to help her win him back, although his plans backfire on many occasions.
Makoto's mother and younger sister, Itaru Itou, get along with Kotonoha very well, which can be largely attributed to her gentle and polite behaviour.
Kotonoha refers Makoto with high honorifics. It is suggested she doesn't care about Makoto has paramours or not very much, as long as he admits he loves her the most.
In School Days, Kotonoha considers Sekai Saionji as her dear friend who's responsible for putting her and Makoto together. Another reason is Sekai actively approaches her and is friendly to her while most other people incredibly cold and even hostile to her.
Sekai also helps Kotonoha and Makoto become lovers, thus she is extremely grateful. That's why Kotonoha is too disappointed when figures out Sekai's betrayal and resorts in unconventional solutions in some cases.
In Two Lovers ending Kotonoha develops bisexual feelings for Sekai as they end their rivalry and decide to share their relationship with Makoto.
In Summer Days, Kotonoha and Sekai dosen't know each other since chronologically Sekai's involvement between Kotonoha and Makoto happens in the second semester, while the story is set during summer vacation, right before the second semester.
In Cross Days there is not much to say about relationship between Kotonoha and Sekai because the latter is reduced to a tertiary role.
In the manga version they get along well,and Sekai accepts Makoto dumping her over Kotohona after the betrayal. She wants to remain friends and watches Makoto apologizing to Kotohona. However at that point Kotohona reveals that blames Sekai for the betrayal and not Makoto.
Setsuna Kiyoura slightly knows Kotonoha since both of them are Class-Representatives (of different classes) and members of student council, however their relationship differs in different stories.
In both School Days and Cross Days, Setsuna sees Kotonoha as an obstacle to Sekai and Makoto's relationshipand actively tries to sabotage their relationship, though her methods are slightly more discrete than Nanami's and doesn't involve bullying Kotonoha. Like Sekai, she also doesn't seem to have negative assumptions about Kotonoha.
In Summer Days, Kotonoha becomes Setsuna's close friend (even closer than Sekai) during one of her routes, and is told by Setsuna not to use honorifics to refer her.
Otome Kato and her followers (Minami Obuchi, Natsumi Koizumi and Kumi Mori) are the main group who bully Kotonoha. They are seen to disturb Kotonoha's school life in multiple ways, including forcing her to do the work of others. They also have negative assumptions about her that are in no way true, which mostly stems from their jealousy of Kotonoha's beauty and the attention she gets from boys.
Otome herself, who also has feelings for Makoto since junior high, goes even further after acknowledging Kotonoha and Makoto became a couple. For example, in a certain route she arranges for Taisuke Sawanaga to brutally rape Kotonoha during School Festival.
However Kotonoha never retaliates against Otome and her gang's bullies, she even retains a polite attitude toward them. She is, however, not afraid of them, and rises to defend the legitimacy of the relationship between Makoto and herself in a courteous manner.
Nanami Kanroji studied in the same junior high Kotonoha did, she was hostile to her since then. She also spreads fabricated rumours of Kotonoha around in Sakakino Gakuen, Otome's hostility to Kotonoha can be partly attributed to these rumours. Since being Sekai's friend Nanami now has one more reason to bully Kotonoha, and participates in sabotaging she and Makoto's lover relationship. Even though she fabricates false rumours about Kotonoha, she herself seems to believe they are true and doesn't even consider the possibility Kotonoha and Makoto are dating and even shamelessly says Makoto is Kotonoha's imaginary boyfriend.
While the bullying by Otome and her friends are at the very best trouble Kotonoha slightly, Nanami can coordinate with them to start bullying her with more intensity, driving her to the point of insanity.
Kotonoha is still polite to Nanami and never retaliates her bad deeds, though can talk back to her at one point.
Hikari Kuroda is hostile to Kotonoha in School Days since she's a friend of Sekai, however in Cross Days their relationship is portrayed to be more friendly.
In Summer Days, the two have little interaction.
Ai Yamagata considers Kotonoha as a rival since herself, like Otome, has feelings to Makoto from junior high. In Cross Days Ai is seen to spread fabricated rumours about Kotonoha, and in a certain route, temporarily gains the upper hand in "Contest of Makoto's love", but though she is defeated.
In one ending of Cross Days Ai cheats on Makoto and Kotonoha by claiming she is pregnant from him, but in fact she's carrying Yuuki Ashikaga 's baby. Being shocked by the "amour" of Makoto and Ai, Kotonoha goes berserk or yandere and reveals her Japanese blade.
In Cross Days, it is seen Roka helps Kotonoha transporting stuff during School Festival, so it is suggested they get along well with each other.
In Cross Days Yuuki Ashikaga initially is aware of Kotonoha since as a library staff, he always sees her visiting library and reading. Believing his crush Roka in in a relationship with Makoto, Yuuki volunteers to help Kotonoha to win Makoto back in order for himself to win Roka back, even by some unconventional methods.
Although his plans backfire from time to time, Kotonoha is very grateful and consider Yuuki as her benefactor, also tries to promote the relationship between him and Roka as return.
However in one of Cross Days' ending, Kotonoha eradicates a Yuuki in guise if Yuu by a crowbar in order to "put an end to Makoto-kun's adultery". In another ending, Yuuk's sister Chie Ashikaga is mistaken as the girl-disguising Yuuki and is eliminated by Kotonoha by the same means.
Kotonoha considers Taisuke as an acquantance from school, but in School Days if the first three episodes are completed in Sekai's favor, Kotonoha will end up in a relationship with him. It happen with Taisuke simply asking her out and Kotonoha accepting because she's explicitly dumped by Makoto, or be raped into it. However in different routes their relationship is far more pleasant
In Cross Days, in two of the endings Taisuke rapes Kotonoha, and in one ending Kotonoha gets saved by Yuuki when she almost gets raped by Taisuke.
In the anime Taisuke rapes Kotonoha but Kotonoha remains loyal to Makoto.