Mirei has a strong feeling of protection towards Mamori and she certainly shows it in the whole anime. Even though Mirei seems like she uses Mamori just for fighting, she does try to pleasure Mamori to initiate Drive and seems to actually like it herself. At some point, even though it's not explicit, Mirei starts to develop stronger feelings towards Mamori and even gets jealous when someone is closer to Mamori. Mirei has stated that she didn't have a reason to live and didn't know why she protected Mamori with her life, but then she realized that her mission to protect Mamori at all costs and her love for Mamori were all she had and that was her reason to live.
Rein has stated that she finds Mirei very fascinating, although Mirei doesn't share the sentiment. One instance, where Rein wanted Mirei to use her as her Arm, the two had shared a kiss, although Mirei was far from pleased in the aftermath of their brief team up.
Like her partner, Rein, Lady J seems to find Mirei fascinating. She did nothing but chuckle when Rein had kissed Mirei to initiate Drive and even explained to her how Rein's Arm ability works.