One element is the lyrics. If the lyrics are about dark topics like death, ghosts, or curses, it can be scary. For example, lyrics that describe a spooky encounter with a supernatural being. Another is the tone of the rap. A slow, deep, and menacing tone can add to the scariness. Also, the setting where the rap takes place. If it's in an abandoned place like an old factory or a haunted house, it gives a creepy vibe.
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.
Well, here's one. In a small town, there was a rap battle. A young rapper named Jay took part. He rapped some really bold lyrics. But after the battle, he started having nightmares. In his dreams, he was in a dark room and a shadowy figure was rapping at him, criticizing his skills. Every night the dream got worse, and the figure got closer. One day, he decided to stop rapping, and the nightmares finally ceased.
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.