Lugh Beowulf from Type Moon's: Mahou Tsukai no Yoru aka Witch on the Holy Night. Lugh is a nature spirit, a being that is bound by no form an can take on any form he so desires. His role in the story is basically as a thought to be answer for bringing the number of werewolves back from the brink of destruction, but as time passed he was shown as ageless and had little interest in helping the werewolves, so he was sold and went on adventures with the main heroine, where he basically killed everything that dared oppose him and by extension her. Reference picture below.