Dungeons and Crayons

At 4, Groo is undoubtedly the youngest S-ranked awakened in the country. But as her kindergarten class's (self-proclaimed) ace, she can take a challenge! So when her first quest window tells her to find a grown-up who'll take her in, she marches down to the top guild in Korea and demands to be recruited. Sensing that this overpowered orphan shouldn't fall into the wrong hands, the guildmaster has no choice but to adopt her. But he has no idea this tiny girl is about to turn his life upside down!

(Source: Tapas)