mimiina and yumaosa have so many parallels yet are so fundamentally different that it's really fun to read lol. not even in a shipping way, just in a "their dynamic are like two sides of the same coin" typa way
mimi and yuma are similar in that their parents gave up their lives in order to protect their kid, giving said kid semi-immortality (temporary, in yuma's case) which is later exploited by the adults around them since they were raised during warring times. the main difference between the two is that while yugo's death and absence is keenly felt and internalized by yuma, mimi's feelings towards her mother's death feel almost muted, somehow. i don't know if this is because mimi doesn't know she's grieving her mother, but since we visibly see mimi learning about grief after seyran's death, it leads me to believe she hasn't learned how to grieve. yuma, on the other hand, doesn't allow himself to grieve. on top of being made responsible for a "loss in fighting power," he was raised with the mentality that decisions that get you killed are on you, which only significantly deepens his self-loathing. also, mimi will permanently live forever unless the magic cast on her is undone, while yuma will inevitably die despite his trion body, so their awareness of death as a possibility to themselves may also be a contributing factor behind their understanding of death and grief. but while yuma threw himself into his goals because he knows he's got little time left so he might as well make the most out of it, mimi throws herself into her goals because she's aware the people around her may not make it if she takes any longer. also, yuma's "fuck it i'll trust and follow osamu" mentality makes him enable osamu, while sheena's "i'm starting to understand the significance of me being unable to die" mentality makes her actively prevent sheena from becoming stronger.
tldr: both characters aren't given the ability to grieve healthily for their parent, and said parents sacrifice to bring their child back from the dead is why both characters end up exploited, which leads to them becoming mentally numb to being used as a soldier and being exposed to the battlefield.
sheena and osamu are a little different in that both of them are innately weaker than the people around them because of factors outside of their control. sheena has weaker than average magic who specializes in the rare, but impractical, recovery magic. osamu, on the other hand, has lower than average trion. both of these handicaps limits sheena and osamu's ability to contribute to the battlefield, and causes visible frustration within both osamu and sheena for needing other people to do things for them. however, while osamu still tries and desperately struggles to find ways to contribute and help, sheena accepts it for a while and though he continues to study, she can't help but surrender to the idea that she is fundamentally useless as a weak magic user in a school raising child soldiers. basically, one gave up, while one was still trying prior to the start of the story. it isn't until they meet their respective partner, however, that they manage to make substantial moves towards their goals within the story. sheena's desire to protect mimi forces her to leave behind her old mentality and start to struggle even if it becomes useless in the end, which leads her to try and find her specialty magic and start training even more in order to get stronger. yuma's presence and bond with osamu, though, allowed osamu more freedom of movement to do things, since yuma's strength has now become available to osamu, broadening his choices of what he can do for the sake of his goals.
tldr: both of them are weak. objectively so. but while osamu still retained a mentality that allowed him to continue pushing onward despite that fact, sheena's world actively prevents her from doing anything similar, which leads to her seeing herself as useless. both, of course, retain a strong sense of morals, which often comes to a head when caring about their partners. said partners' presence also allows them to significantly change both mentally and, well, combatively.
in conclusion: mimiina and yumaosa have individually gone through such similar events to each other, yet their fundamental differences stem from the characters' individual reactions to said events that wind up defining their combat potential. these differences, of course, end up informing their partnership with each other, which seems similar on the outside, but are different in very defining and fundamental ways.