The Secret Angel.

An angel called L accidentally saves Han Si Yeong, who was meant to die, when she gives him her necklace. Now it's her mission to live on earth and protect Si Yeong for 100 days, while an angel of death tries to manipulate him into taking his own life. If she fails, she'll be banished from heaven forever and stranded without her powers. However, the more time the angels spend on earth, the more human they become, and with that come human feelings.