Tamer is a profession that can tame monsters and use them as familiars. That was the vocation of the main character Kohaku, but he couldn't tame ordinary monsters. As a result, he was called "incompetent" and banished from the guild. However, he had a special power that allowed only him to tame the mythical "Phantasma", a legendary species so power and so rare that they have never been seen in public.

(Source: Comic Nova, translated)