Alastor and Lucifer maintained a relationship of "exchange," meaning they did not love each other but acted as a couple. Alastor saw no problem with that; after all, he is asexual and wasn't 100% attracted to Lucifer, he was simply seeking power. But Lucifer felt an emptiness in his heart... He knew it was only about power and nothing more, but if this was the closest thing to love he could have with Alastor, so be it...

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Lucifer was making rubber ducks; he wanted Alastor's presence in an affectionate way, but... they both knew it wouldn't work.