Garo S1

Garo focuses on the life of Kouga Saezima, who has assumed the title of Makai Knight to protect humanity against dark demonic manifestations called "Horrors"
In his quest to destroy them, he encounters a young girl named Kaoru, whom he saves from a Horror, though he learns that she is stained with its demonic blood

As a rule, those that have been stained by the blood of a Horror must be killed, or else they will die painfully in approximately 100 days
Kouga spares Kaoru and tries to find a way to purify her before her remaining time expires
Thus, the series focuses on Kouga's developing relationship with Kaoru, and his responsibilities protecting humanity in accordance with the wishes of his father, the previous Garo

In the process, he encounters another Makai Knight named Rei Suzumura, who eventually becomes his ally
Later Kouga confronts his father's former disciple who is revealed to be the cause of a recent series of Horror attacks in preparation of a more sinister advent of the Horrors' originator, Messiah