He was an orphan and the knight that wielded the Silver Fang armor at that time took pity on him and raised Rei as his son, training him to inherit the title and start his own lineage, continuing the tradition of guardianship.
He was an orphan and the knight that wielded the Silver Fang armor at that time took pity on him and raised Rei as his son. His father (the house butler) was actually an old dying Makai Knight who never had kids so Rei was trained by him to inherit his title and start his own lineage, continuing the tradition of guardianship.