The ending was made so to satisfy readers and make them want to read more, the novel probably covers all the manga in the first volume, or in 2, and the other 3 have other cases.
Hard to know without reading if MC keeps being the center of the story or it switches to anyone that Riaki is tormenting.
Given that girl I can guess MC keeps the role and something happens around him.
Maybe one day the novel will be translated until then I cheer for prez as girlfriend, she have her problems but nothing crazy.