Ori no Naka no Soloist

Chloe and her baby brother Locke were born in a prison the size of a city: a walled-off collection of ramshackle buildings full of dangerous criminals who live bleak and isolated lives. Their parents are gone, leaving Chloe to care for her brother. When a trio of soldiers manage to break out of the prison, Chloe goes with them but loses Locke during the escape. After years of training, Chloe returns to the prison city with one goal: to find her now 11-year-old brother and get him out of that hellhole, no matter what it takes.
(Source: Seven Seas Entertainment)