Unexpected in this type of show, but it was hard not to have your heart melt a bit at the end of "Soul Sucker." Callie ripped out her human soul to sacrifice to the Puzzle-Box Mark had gotten her, since he'd backed out of the deal. Mark then shows up at the last second, approaches his very demonic-looking girlfriend, and comments on how her soul is the cutest thing he'd ever seen, places it back inside her, and volunteers his soul to be sacrificed. Callie then proudly states that she's a half-human demon, and decides not to go through with it.
And then Leonard prompts Mark to dump pig's blood on Callie, which is the "sweetest thing anyone has ever done" for her.
Referenced near the end of "Callie and Her Sister" in Season 2, where Callie gets pissed and goes full-blown demon (handing Mark her soul for a bit) when Mark informs her that her sister, Lillith, intends to kill her and Twayne so she can marry Mark instead.
And subverted about 3 minutes later, when Callie tells Mark she wants him moved out of her apartment by the end of the weekend.
Debatable of course since he's the leader of Hell, but Aldramac deciding against naming Callie as his successor after seeing the look on her face and knowing that she's spent the entire series not wanting to follow in his footsteps.
Mark's first use of his power over free will in "The Ring Of Powers" is to tell an aggressive pedestrian to have a wonderful day.
In "Mark Loves Dick," this troper finds it really heartwarming that Mark decided to stay with Callie even though she was a man.