Nope, Marin and Gojo don't end up as a couple in the manga. The manga follows a different narrative path for their relationship, concentrating more on other aspects of the story and character growth.
Since I haven't read 'mujhe sandal kar do by nimra noor', I can only speculate on the theme. It might deal with the experiences and emotions of the characters. It could be a story of love and loss, or perhaps a journey of self - discovery. The title itself doesn't give away much, but it could be related to a specific event or action within the story that has a deeper meaning in the context of the characters' lives.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.