WARNING! THIS CAN CONTAIN SPOILERS!!
Heartbeat Cupid is the security guard of the Sin's Apartment, he got no special abilities and he has incredible shyness and anxiety.
A long time ago he had a heartattack and died immediately, but before he was found by anyone who knows what the fuck to do with a corpse, he was found by the sins, Gluttony to be precise. When Gluttony found him he carried him back home to the Sin's Apartment and they penetrated the side of his chest and his heart with a crank and by cranking it he came back to life, also his hair for some reason turned from blonde to pure pink. And from this, he gained amnesia in which he forgot who he truly was, and therefore he gained the new name Heartbeat. He is around 20 years old.
He has to spin the crank to keep himself alive and if he were to stop he would die until someone would help him spin it again.
The first person he saw when he came back to life again was Wrath, and ever since that day he came to life again, he has been madly in love with her. (Wow, a innocent, shy boy falling in love with Satan herself, interesting.)
Heartbeat is (like most others in this show) A Dumblotraight. He is mint boned.
The voice actor for Heartbeat is Leeroy Ellsworth