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Rukia as the Mother. Yes with a capital letter.
Is a subset of both Rukia as the goddess and Rukia as the family member.
She saves his life. She gaves him a new life. He is symbolically reborn on her sword.
She protects him, and that’s why he wants to protect her, just like he wanted with his mother when he was little and Tatsuki was kicking his butt at the dojo.
She gives him a second life. She gives his life meaning. She gives him the tools to deal with his past while respecting his boundaries. She helps him realize what his goals are are she is lending him her own power to achieve them.
He is abrasive and at times seems even disrespectful towards her, but he is the same with his father, another violent mentor figure that keeps him on his toes.
She makes him choose the right path, always believes in him and puts his safety and well-being above her own.
If she dragging him to apologize to Orihime after the Yammi incident at the park, is interpreted as Rukia shipping the two, she even appears as the fairy tail mother- in-law that wants a pushover that worships her son for a daughter-in-law ; )
She is a protector, mentor, friend. The older, more experienced, holder of knowledge. In other words she is a Mother figure.
Maybe all those parallels between Rukia and Masaki were intend in more literal sense than we thought, maybe Rukia is really there to function as a mother figure. It would not only explain why no ichiruki, but why they spend less and less time together as the time passes and he establishes new functional relationships.
Against it, among other things, stand Rukia’s own emotional stunted growth, the fact that she is very young for her species, and the fact that he is not comfortable in accepting her protection.
Not really sure about a mother, but maybe the author wanted a big-sister archetype. Unfortunately for him, he created by mistake a love story. If I was him I would have just run with the love story, alas I’m a talented nobody writing manga essays on the internet, regretting how one of the most compelling relationships ever written ended in the kind of friendship where the two see each other about once every decade.