Sei is a shinigami who is attached to Miku until her life ends.This is due to some problems in the Shinigami World. Miku was not supposed to die but due to an error, Sei was sent to retrieve her soul. He then placed the Thanatos Lover on her which won't allow her to be parted from him until her death day.
Although Sei is a shinigami, he his very kind to humans. And he also seems to have a sweet tooth. When he and Miku went to visit Jun, he ate his share of the cake they brought her and her share as well. Jun realized that he loves sweet things and offered him a chocolate cake her relative gave her but was too sweet for her liking. He gladly accepted it.
Another time, Sei and Miku were at a cafe when he suddenly licked her face. He then explained that he nevers lets any sugar go to waste.