Kaichou-sama no Niseyome.

Becoming the fake fiancée of the popular, playful student council president!?

Tachibana Mayuka transfers to a boarding school. At that school, the incredibly handsome but playful Yuki Nozomi reigns as the student council president. By chance, Mayuka becomes Yuki's fake fiancée!? She's just pretending to be his fiancée... even though she doesn't really like him... Mayuka finds herself increasingly drawn to Yuki, who is selfish and perverted, but sometimes kind!? A tsundere high school girl is forcefully loved by a playful, self-centered guy in this sweet and spicy fake marriage love story★

(Source: Shogakukan, translated)

Note: Includes four extra chapters.