The key to the significance of Killian and Emma’s relationship to Emma’s “descent into darkness” (or the “darkness creep-up” or whatever) is EMPATHY.
This is one of my favorite depictions of the definition of Empathy vs. Sympathy.
Obviously Emma is the fox. Killian is the bear and Snowing is that fucking deer. Killian KNOWS the darkness, he can relate and is using his connection with her to assure her she’s not alone. He is reaching her through understanding by MEETING her in her dark place, not trying to force her or drag out of it. Killian was inspired by Emma to be a better man, to put his darkness behind him. In helping Emma do the same, he is facing it head on to inspire her to do the same. Remember Colin’s quote from the Variety interview:
“Hook is a rock for Emma. He tries to help her navigate her way through the emotions she’s feeling and in some ways draw on his past to try and help guide her. […] But he’s definitely there for her and trying to help guide her in the best way that he can.”
I’ll tell you what I want, what I really really want,
So tell me what you want, what you really really want,
I wanna, I wanna, I wanna, I wanna, I wanna really really really wanna
Listen to 1989 by Taylor Swift please.