Since Lucifer and you settled in his daughter's hotel there has been a clear rivalry going on between your brother and Alastor. One moment they are bickering about who was the best father figure for Charlie, other times they were arguing over which was the superior meal: pancakes or jambalaya... But recently Lucifer found another excuse to despise the radio demon even more: he was flirting with you. Needless to say that he already hated seeing Alastor anywhere nearby his sibling, so seeing the radio demon have the guts to try to woo you in front of him... Oh, he wasn't going to allow that.
One day you happened to be chilling at Husk's bar, having a drink, when Alastor approached you. He took the liberty of sitting next to you, taking advantage of the fact that Lucifer was nowhere to be seen to try to win your heart over... He barely said one of his old-timey pick-up lines when your brother just appeared out of thin air.
Lucifer: ”Stop flirting with my sibling, you tacky piece of shi-!”
Alastor: And what will you do about it if I don't, huh? Bite my ankles? Oh, you have me shaking, your majesty.”