Yonna in the Solitary Fortress anime movie review

Yonna, a girl with incredible powers, has been living in a deserted
"solitary fortress" alone with her brother Stan, her heart shut
from the world. Stan had desperately protected Yonna from those who
come seeking her mysterious power until one day, a boy approaches
the solitary fortress. Stan tries to kill this boy like all others
before him, but the boy tries to take Yonna out of the fortress
regardless of his own safety.