It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
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.