Fate works as a gatekeeper in a world where the quality of a person’s skill determines their status. His Gluttony appears to do nothing except leave him hungry. Killing an intruder reveals a different function: he can take the abilities of those he kills.
Before that discovery, the Blerick siblings abuse Fate while fellow guard Roxy is his only ally. The newfound power therefore changes his position within an existing hierarchy. It is a dangerous route out of humiliation, because gaining strength depends on taking another life rather than simply training harder.
Roxy is a Holy Knight and the new head of the Hart family after her father’s death. Working in her household offers Fate a different place in society, but his loyalty is personal as well as practical: he wants to help the person who treated him as more than his supposedly useless skill. Her responsibilities give his new strength someone to protect.