SICK: Watashi no Tame no Kaibutsu (Light Novel) - Recommendations

Takes place in the Kyushu island region. Nagasaki Prefecture, in the early twentieth century, was a magical kingdom whose imminent power prompted the Americans to drop the atomic bomb on them in World War II. In current times, Nagasaki is full of magicians and is called the Land of Magic, but has isolated itself within a walled city. The magicians on the outside must keep their abilities a secret, and are limited in that they cannot cross the ocean. Mizukura Risuka, a ten-year-old girl living in nearby Saga Prefecture, is the daughter of one of the most powerful wizards. When she is not attending school, she searches for her father. Her magical ability allows her to transform her physical age; when she is about to die, she can transform to her prime physical age of 27. She teams up with Kugi Kizutaka, a ten-year-old class representative, who is a genius and a sociopath with desires to rule people, treating everyone he meets as pawns. Kugi is termed a "Mage User", although he has no intrinsic magical abilities.