Family
Rei
Remi admired her older brother's altruism and tries her best to follow his example, regardless of the disapproving opinions of others.[1] Judging by her saddened reaction after coming across the deathpool flyer at school, it is highly likely that Remi had a close relationship with her brother, and additional art from Uru-chan suports this.. Rei's death shocked Remi to the point that she became more passive.[2] Furthermore, Remi's hatred against EMBER stems from her desire to avenge her fallen brother.
Mama Lightning
Although they are not seen interacting much on screen, Remi presumably shares a very close relationship with her mother, just like Rei.
Wellston Private High School
Blyke
Not only are Blyke and Remi teammates, but they are also best friends. Blyke especially appears to genuinely care for Remi as shown by his concern over her changed personality and tends to be very protective of her well-being, even if it means taking risks for himself as shown during his battle with Volcan. It is also implied that Blyke may be competing with Isen to win her heart.[3] Remi, in turn, also cares very deeply about Blyke. She burst into tears of when she learnt that Blyke was secretly a vigilante, worried for his safety. However, she also enjoys messing around with Blyke in lighter moods, whether it's chucking a ball at his head or drawing on his face while he's sleeping.
It should be noted that Blyke made X-Rei's costume in order to support Remi's time as a vigilante, and the two are currently formulating a plan on how to counter Joker. Though Blyke does not always see eye-to-eye with Remi, he always tries to his best to support her in any way that he can, and at times, shares her stubborn drive and determination.
It is also likely that the two of them have mutual feelings for each other, as is evidenced by their concern for each other, and the closeness of their friendship.
Isen
Isen is one of Remi's closest friends and can often be seen hanging out with both Blyke or Remi. Of the trio consisting of him, Remi, and Blyke, Isen is the most self-preserving of the group and often has to remind the more headstrong Remi about the potential dangers of the situation. Despite his self-preservation, Isen has saved Remi when her life was on her line during her battle against Volcan and attempted to claim that he spied on John out of his own volition, in hopes to keep the Joker's wrath away from her.
Though Isen's self-preservation and Remi's obstinance tend to make the two butt heads, Remi still views Isen as a valuable friend, and is not above playing practical jokes with him, as shown when the two doodled on Blyke's face.
Seraphina
Seraphina and Remi are teammates and acquaintances. Seraphina is grateful that Remi took her place as Wellston's Queen and Remi is grateful that Seraphina gave her the position.[4] Like other students of Wellston, Remi views Seraphina as above her.[5] However, Remi's admiration for Seraphina is more than skin-deep, and she is shown to maintain a deeply positive relationship with the now-crippled Ace, going as far as escorting her to the infirmary and warning her fellow high-tier about the tracker in the Teddy Bear.
John
While not necessarily close to John, Remi saw John as Seraphina's friend and was more willing to help him than other students. However, her first interaction of John caused him to violently slap her.[6]
When she learned that John was the Joker, Remi views John as someone who must be stopped due to his violent actions. She attempted to reason with John in the hopes that his rampage could be brought to a peaceful end, but her pleas were met with hostility. All her words about royals working hard to make the school safe for everyone only served to anger John, whose told her of his experiences as a victim of the student body. His rejection of her call for peace frustrated her, and she resigned to meet him in combat as he planned.
Remi soon came to realize that she couldn't deny John's words about Wellston High being an environment where the strong constantly prey on the weak. Even so, she still found his actions as Joker deplorable, believing he was only making things worse as Joker. She attempted to put a stop to him by exposing his identity to the school, though this ended in failure. Remi's relationship with John only worsened when the latter, now King, launched frequent attacks on the Safe House, and refused to co-operate with her.
However, after John atones for his violence, she warms up to him.
Arlo
Remi has known Arlo before she enrolled into Wellston through Rei. [7] Their tenure as the King and Queen of Wellston was not one of cooperation due to their conflicting philosophies as to what their roles should entail. Remi took after her brother, and tried to help her fellow students and keep them safe. On the other hand, Arlo was mainly focused on maintaining his reputation as the strongest (male) student of the strongest school in the region. He only interacted with other students to prevent them from trying to "rise above their station." Remi disapproved of his prioritizing strength, which caused some strife when it came time to choose a Jack. They managed to minimize these ideological clashes by operating independent of one another. This worked against them when both got in over their heads when trying to achieve their goals to the fullest extent (Remi with EMBER, and Arlo with John).
While they have their disagreements, they still care for each other immensely as Remi reacted with anger and worry when she found out someone managed to wound Arlo. He in turn assures her not to worry but is glad she cares. Arlo in turn worried when he heard of the mall incident. He teases her and calls her “Pinky”, and he probably willingly gave the last piece of cake to her on a whim. Both have immense respect for Rei (albeit for different reasons), and both try to comfort each other following his death. When Remi refused to follow Arlo's advice to take a dive during Joker's attack on her, Arlo resolved to step in and fight to the bitter end alongside her.
Others
Valerie
Valerie is Arlo's aunt, as well as the woman behind the moniker of Volcan. Remi put the two and two together after they met at the cafe. She pointed this out to Arlo, who did not take the information well. After Kuyo told Remi that she is one of the people that killed Rei, she vowed to take Valerie down. She relayed this to Arlo, which ended up straining their relationship even more.
Kuyo
Kuyo is a friend of Rei and a fellow vigilante. Like Arlo, Remi knew him through her brother. She would later find out about the circumstances behind Rei's death from Kuyo, strengthening her determination to avenge her brother.
Grayson
Grayson is another vigilante that Remi encountered during one of her outing with Blyke, Isen and Kuyo.[8] She rejected his application of joining their team.[9] Nonetheless, she is grateful for his help despite being somewhat apprehensive of his infatuation towards her.[10]