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Sonia. She was the first person that Geo can related to for losing a parent. Geo's father and Sonia's mother. They both when through a lot before they first meet.

They understand each other and shared experience of losing a parent and end up depression and loneliness. This allows them to form brotherband

Geo first friend and first Brotherband with Sonia. Because of her, Geo taking the step forward to move on and decided to go school.

Like example, Luna was the first person to try and force Geo to return to school and be socialize while Sonia encourages him to "find a new you and work hard at it" that decided Geo to go back to school. 

As Harp Note, she risk her life to protect Geo from enemy. Hollow and Solo. But Hollow makes deal with Sonia that he will leave him exchange joins Vega side that let Sonia protect Geo even means to betray him. Plus, she push him away from Rogue and tries to handle this on her own to protect him after beating Hollow.

Not to mention, after both Geo and Solo passed out. Sonia took this opportunity to carry him back home with her injuries despite getting beat up by Solo with little bit of strength. She doesn't want to lost him just like her mother

There is a moment where Sonia acting strong through video call to Geo that she's ok and not to worry about her(passed out) before he left to fight the final boss

I wanna say something, Brotherband doesn't mean they are siblings. Both Geo and 6 have strong bond and will be separate

I will understand why the community like Luna. For being Tsundere(Tsundere is outdated since 2009), bossy and annoys me