Marriagetoxin

Hikaru Gero belongs to one of the underworld's most feared Poison Master families and has spent his life learning how to kill efficiently, not how to date. When the family head decides that Hikaru's sister must produce an heir to preserve the bloodline, Hikaru vows to protect her by taking the succession problem onto himself. He needs to get married. The only person who can teach him what he knows nothing about is Mei Kinosaki, a brilliant marriage swindler who was originally the target of his latest job. An assassin who understands poison better than affection and a con artist who understands romance as a business form an unlikely partnership, while the criminal world makes sure Hikaru's search for love is anything but normal.