Marriagetoxin

A hitman and a marriage swindler set out on the world's most grueling battle for marriage! Gero is a young man from a clan of hitmen, which has been in existence for hundreds of years. As someone who's never gone to school and lived his life dealing in the dark side of the business, he considers himself to have "no prospects with women" and marriage as "something not for him." However, one day, in fear that their lineage would dissolve, his clan warns Gero that if he doesn't marry and produce an heir, they would have no choice but to...force his little sister into doing "that." Driven to his wits' end, Gero asks the target of his work, a marriage swindler, to...

(Source: MANGA Plus)