Imigo toshite Suterareta Shoukanshi, Jitsu wa Sekai Saikyou

A mysterious figure who commands spirits beyond comprehension... a summoner. Born with a skill loathed by the nobility, Rybert has grown up despised even by his own parents. No matter how hard he tries, his efforts are always ignored. The only person who has ever believed in him was his younger sister, Azria. But when his sister becomes the victim of a bandit attack, his parents blamed it on him and cast him away. Consumed by despair, Rybert collapses in the streets, where he is found and taken in by Sasha, a guild employee. No longer bound by his torment, his powers as a summoner start to awaken...

(Source: Futabasha, translated)