Her Way to become an SS-rank Mercenary

Theon, a mercenary who had a dark past and shady personality, aims to be the youngest SS-rank mercenary to surpass her grandfather. To become the strongest, an official of the organization gave her a special mission. It is to eliminate the head of a mafia, Qailer Mordeno. She undercovers herself as a nanny of her target's sole heiress, Sally, to infiltrate their heavily secured mansion.Will she succeed in her last mission or the least she wanted, fell in love.