Tenningo

Akesagi is a princess so strong that others refer to her as "Onihime"
Her brother, in contrast, is sickly and weak - but he is the one set to be their father's heir until their castle is crushed following the betrayal of one of Akesagi's companions.
All at once, everything changes - her parents are killed, her brother is seriously wounded, and Akesagi finds herself nurturing the flames of revenge in her heart as she is forced into hiding.
Then, in the forests, she meets the mysterious boy Hahoya...
(Source: Ephidel, based on the summary at the nekohigesensei blog).