I like to think Arlene is a stoic type who takes no bullshit. She's not looking to one-up anyone in a snarking competition or a monster-slaying competition, or whatever. She rolls her eyes at Christian's impatience, and when he crosses the line, she snaps at him. She's observant but minds her own business and the business of those she loves. She is... maybe a little like Susan!

Christian is the most suspicious of Samuel. (Maybe that's why he gets possessed.)
This makes so much sense. And do you realize you almost have a completed fic here? You should put it all together and post it. I can see it ending after this last line of yours if you add something to the effect "and that's when it all starts going downhill" or something, flesh out this section a little bit and voila, the fic I've always wanted ever since we met the Campbells. You're magic.