Salem has been a demon of Lust for as long as the Fallen Angels have been in Hell. Originally offered to Lust by Lucifer to be a spy, Lust and Salem did not get along. Lust kept Salem close, though, as one does their enemies. Salem was treated to several millennia of grunt work, erotic torture, and other forms of depravity before Lust was sentenced to reincarnation. Lucifer did not want her back, and so Avalon took Salem under her wing.
Alone with Avalon, Salem leveled out. There was only so much flirting Salem could do with an emotional wall (Avalon) before Ero and Myka came into the fold. By then, Salem's moral compass was (mostly) back on track and as Myka's designated babysitter, Salem took their mutual loyalty for Avalon very seriously.
Salem has not spoken with Lucifer since discovery during Lust's reincarnation court hearing.
Salem's arc very explicitly revolves around the idea of people who approach relationships with a sex-first mentality. Avalon was only the start of Salem understanding boundaries and Laurie is the final boss battle on Salem's path to a friendship-first mentality.
As a true, full-blooded demon, Salem is fiercely loyal first and foremost to her Prince. Despite passing hands several times, Salem was forged by Lucifer, the primary Fallen Angel. She will always choose Lucifer if given the chance and secretly views her pairing to Lust as both an obligation and a slight because her creator has always hated Lust. Lust has always had a hard time coping with having Salem as her assigned right-hand person and loves her as an extension of Lucifer.
Salem is a politically valuable gift that both Lust and Lucifer recognize.
