Sebastian is a character from Black Butler. He is a demon who made a contract with protagonist Ciel Phantomhive. In exchange for seeking out revenge for Ciel, he will have Ceil’s soul when the task is done. Proof of the contract is in the form of a mark. Sebastian’s mark is on the back of his hand. Ciel’s mark is on his eye, which he conceals with an eyepatch. As a demon, Sebastian is known to be a ruthless and skilled killer.
Naruto Uzumaki from the series “Naruto” is an adolescent training to be the most skilled and respected ninja in is village. Kurama, a dangerous fox-like entity is sealed in his body by his father. His father was the leader of his ninja force. Naruto and Kurama eventually earn one anther’s trust and share their strengths.
As an alternative if you’re not into ninja anime:
Set in the year 2021, the human race is plagued by Gasttrea, a parasistic virus. They live within the monolith walls created from Varanium, a metal that is able to control the Gastrea. Children born with the virus obtain superhuman abilities, leading them to be called the “Cursed Children”. Rentarō Satomi, a high school student and Promoter in Tendō Civil Security Agency carries out missions to prevent the destruction of the Tokyo Area and the world.