Sarina at first used to truly despise Aya, bullying her daily for years on end, and had no remorse for her actions whatsoever. She held a grudge on Aya for killing Erika and Shota and swore to kill her no matter the cost. When Aya saved her from the collapsing ceiling her feelings towards Aya changed drastically, and started to rethink her actions towards her. In the later chapters she starts feeling remorse for Aya and has even started to get along with her a little, even offering her own lifespan to give to Aya.
After knowing Tsuyuno used her stick to give her a scar on her neck, Sarina is furious about this and became a magical girl to avenge Erika and for what they did to her. She then follows to Yatsumura's apartment and tries to kill Aya and Tsuyuno. Later she saves her along with Aya and Rina by using Aya's stick.
While there isn't much interaction between these two, Sarina mistakens her as her actual sister in the hospital. She later takes Shioi's place as the Magical Girl Hunter, who steals and uses their sticks. Once Shioi survives, Sarina shows discomfort and awkwardness as she is disguised as her older sister. After learning about her being a magical girl, in later chapters, she tells Shioi to not disguise as her, but Shioi refuses, since she likes how she looks. When Rina tells a bad comment about Saki's body, Sarina will tell her to knock it off.
Sarina told Nijimin where Shioi was located so Nijimin could avenge her friend, Mikado. Nijimin later tells Sarina that there's a barrier around Shioi, preventing her from murdering Shioi.
Sarina, Erika, and Kawano are a group of friends that bully Aya Asagiri, though their past was not shown in the manga it is possible they were childhood friends. Sarina and Erika might be more closer since Sarina said that Erika was her best friend and was really angry about her death. Erika's death was the reason why Sarina's hatred for Aya grew but it calmed down.
Not much is known about their relationship but they seem to be pretty good friends as Ai joined Sarina and Erika to bully Aya. Ai is also in a picture with Sarina and Erika.
Not much is known about their relationship but they do have a junior-senior relationship. Shota asked for Sarina's approval if it was okay to take Aya's virginity, meaning Shota looks Sarina higher than him. When he and Erika died, she was also shocked about his death.
Not much is known about their relationship but it seems that Saki cares and loves Sarina so much. She might be oblivious about Sarina having a mean personality.
Sarina first met Nana in the hospital. Nana told her about the situation and offered her to become a Magical Girl to be the new "Magical Hunter". Sarina accepted and did so, she used her knowledge as her advantage and attacked Aya and Tsuyuno. After she was saved, she decided to collect their sticks in the place where the apartment collapsed. Then they met in a park, where Sarina told her that she got the sticks but questioned her about the contradiction to escape the Tempest. Nana told her that she knows too much and shoots her to death with her invisible bullets.