When an angel and a demon get married, they're referred to as "Lovers"... and a lot of kids look down on them for it. That's how Micael's life started; his parents were Lovers, and he got teased a lot for it, though he did find a friend in Cross, also the child of Lovers. From the start, Micael didn't have an easy life. Even his mother was a little scared by him and his creepy deep blue eyes. His parents also had very little time for him, and he was often lonely.
Then he met a wingless angel named Erinoa. Erinoa had time for Micael and was nice to him...but Erinoa also cared deeply about his little sister Elmelia, who had her own problems (like being in love with Satan), and Micael felt neglected. So...Baal blackmails Elmelia, she falls into Hell and dies, Erinoa blames Micael for it and attacks him (and I think he's also under Baal's influence at this point). They're on a cliff...and when Micael dodges away in self-defense, Erinoa falls off the cliff and dies. Micael's still just a kid, maybe 10 years old...so it's not surprising that these events really upset him. So he was even more gloomy and creepy when he started middle school...which is where he met Filla.
Filla thought that Micael should become an angel, but Micael insisted that he hated angels and certainly didn't deserve to be one...but he thought that Filla should become an angel. They made a bet: if Filla could get Micael to smile by the end of the school year, Micael would become an angel. If not, Filla would become an angel. Well, Filla won the bet. Micael became an angel when he grew up, Filla became a demon, and they became partners. Now, in the first volume, they're sent on a mission to recover one of the deep blue gems called the Sky Blues...