Katsuki is a very aggressive person and bullies Izuku Midoriya, who he also treats in some ways as a rival. He has a superiority complex and wants to be first, to be the best at everything. He doesn't like when people look down on him, as if they were superior to him, and gets mad easily. He is extremely arrogant, but also very smart and capable. He also thinks of people such as Izuku Midoriya as an obstacle on his path to becoming the greatest hero. (Source: Boku no Hero Academia Wikia)

Ruka is born with an irregular heartbeat, so she wants to do exciting activities to make her heart beat faster. She becomes a rental girlfriend in hopes of finding love and a faster heartbeat. While being rented out by Shun Kuribayashi, she meets Kazuya Kinoshita. Her heart beats faster when she's with Kazuya, and she realizes he actually cares about rental girlfriends. She falls in love with him, and confesses her love to him. As his girlfriend, she struggles as she knows him and his family prefer his rental girlfriend Chizuru "Mizuhara" Ichinose. She tries everything she can to make Kazuya fall for her instead. (Source: Kanojo, Okarishimasu Wiki)