Hakoniwa no Margris

A world where two worlds exist: Grimoire, where wizards live, and Grand Earth, where humans live. Theo, a boy who is human who has a special constitution, visits the magical city of Grimoire to learn about himself. There, he meets the great wizard Lucas, who Theo decides to talk to, but his physical condition causes a terrible situation. Furthermore, he learns Lucas's shocking secret――!? A story of the bond woven between a lonely wizard with a secret and a boy with a special constitution. (Source: Zero-Sum Online, translated)