He is very much relevent to the story. You missed the point, the power play that went there.
Her throwing him was basically her last attempt at making him feel like he is inferrior to her. Rage and hatred at herself and him, the littereal son of the person she looks up to, being a reminder.
Him pushing her away but then diciding to engage anyway was basiacally him telling her he won, that he saw right through her, that when it came down to it, she still cant stand up for herself and is indeed inferrior to him. But this is also in a way him forgiving her. So reconciliation in a twisted way from both parts. The last blow from him was ofc him telling her that it is actually her mother who despite all was superior compared to his mom Arisa idolizes so much, who threw herself at other men, getting disowned even, due to her inferriority complex. So his mom aint really that what Arisa thinks and that Arisa should start working on herself
P.S.: also her giving in into the physicality was in a twisted way of her "absorbing" him (instead of his mom, so the aunt), his superiority, the being that she wants to be but can't
why does it have to be included
AND THEYRE TREATING IT LIKE IT WAS NORMAL AND NOTHING EVEN BAFFLES ME. moreover he even seems like not in the story, because the point across is her and her mom right? damn