Mana cares a lot for Shido, her older brother, even though they have not known or met each other since the time when they were children. This is all despite the fact that they don't even have any memory of those times when they were little.
She is protective of him and criticizes Kotori for letting him join an organization that makes him establish contact with the Spirits. At the same time, she seems to actually believes herself to be the one who should be taking care of him, despite being the younger sibling. Much later, after the Kurumi incident, she and Shido reunite during an assault on DEM's Japanese HQ and co-operate to rescue Tohka. Amusingly, not only was Mana aware of the fact that Shido dressed like a girl named Shiori, she was completely okay with it so long as he hadn't actually turned into one and offered, without Kurumi's teasing undertones, to even go shopping with him, much to his confusion and dismay.
They are rivals, obviously due to their 'older brother', Shido. However, after her defection from DEM, the relationship between them has gotten better. After she found out that Kotori was a member of Ratatoskr and that she'd pulled Shido into the group, she berated Kotori for it, noting that she failed as a sister for getting Shido involved with something as dangerous as the Spirits without even a weapon to defend himself. She later prevented a hypnotized Kotori from killing Shido and brought her back to her senses, a thing Kotori is extremely thankful for. They appear to acknowledge each other as sisters, or at least Shido's sisters, and care for each other, with Kotori even begging Mana to get her body treated.
Mana shows a deep disgust of Kurumi’s very existence. She has repeatedly killed Kurumi's clones in battle, however, the real Kurumi does not even consider Mana a threat and doesn't hesitate to prove to her that she is vastly stronger than she is. After defecting to Ratatoskr, an organization specially made for dealing with spirits peacefully, Mana still notes her intention to defeat and kill Kurumi when she gets the chance. She immediately blasted Kurumi's head off on sight during the attack on DEM's Japanese HQ when she saw her near Shido (she assumed she was trying to hurt him, which is justifiable since she knows Kurumi wants to eat him), however, after hearing that Kurumi was on their side for the moment, she begrudgingly chose to accept it.
As Jessica was dying, Mana admitted that she truly hated Jessica for her jealous attitude when she was still part of DEM. Even then, she still offered some comforting words to the former Adeptus 3 as she was dying, telling her a white lie that Westcott was proud of her so that she would be able to rest in peace.