Yuri and Nightfall were both created for somewhat the same purpose. Characters just to add chaos and tension in the Forger's dynamic. For that reason their entire identity was formed around a specific Forger. In a similar sense, Emile and Ewen also existed as auxiliary characters to Damian not much individuality assigned to either.
Normally, these types of characters would be bound to these minor roles and never grow independent of the other characters. Instead, in the last three chapters, we see an intentional effort on Endo's side to continue their growth and development into individuals.
Emile and Ewen started as little echoes for Damian, following him around like tiny mindless goons. Now, although they still exist as a duo and their role is still very tied to Damian, they are shown to have a genuine relationship with him. They're not just in it for the clout, Damian is their friend and they truly care about him and all his flaws.
Nightfall and Yuri are obviously more complicated.
Yuri has a very troubled relationship with his sister. Their shared past resulted in him being very codependent on her and building his entire identity and sense of purpose around her. Now that his sister is married and moved on (i will not expand on that), he is left feeling like an empty shell of himself. He now is a position where he has to find a new sense of purpose while also questioning the choices that he has made throughout his life for the sake of his sister.
The chapter ends with him accepting that his sister is happy and there is nothing more he can do for her, that their lives have forever changed and they cant go back to the way it was. Their home is in the city now and he has to come to terms with what that means for their relationship. In the end, he decides to go out with his coworkers and try to live a little, outside of sister.
Not much is known about Nightfall. Her motives are still ambiguous and defined around Twilight. I have my own personal explanation but the canon still dances around the topic. While her past is still ambiguous, her present circumstances are clear. She is very devoted to Twilight to the point where she routinely risks her health for his sake. She has this idea that for him to love her she needs to be absolutely perfect and should never show any weakness. A moment I found funny was when she needs to cough and blow her nose but refused to do it in front of Twilight. Reminds me of a short story I read for one of my classes, Bride by Cheerie Jones.
So much of what Jones described as the perfect wife is the same things Nightfall tries to do to gain the romantic attention of Twilight. It also shows why she resents Yor because beyond the fact that she has Twilight she doesn't have to be perfect for him. Yor rattles everything that Nightfall believes about herself and again leaves her feeling empty and confused. Again, there is some optimism to her story. Seeing that Twilight still cares for her even when she is not perfect, that his concern for her is not contingent on her being perfect. She is finally able to rest and recover once he leaves her with that simple expression of concern.
I don't know what motivates her or why she specifically craves Twilight's affection but my most optimistic theory is that he represents someone that she wants. Someone who cares for her. Twilight does care for so I am guessing her drive is less to get his care but to preserve it and earn it. She is just seeking his validation the best way she knows how.
