Merlin(Jak)

Merlin, also known as Magus of Flowers, is a powerful Magus, a human incubus hybird, who was not only Artoria Pendragon's adoptive father, but also her  teacher. In his youth he served Uther Pendragon, Artoria's biological father, and later agreed to raise Artoria as his own daughter so she could inherit the throne one day. His real name is Merlin Pendragon and he is the grandfather of Arturia, although she never learned about this up to her death.