Nakimushi Majutsushi, Clear Go no Isekai Life

Rei, a magician torn from his world, is thrust into a quest to slay the Demon King—a journey that wears him down, body and soul. In the midst of his quiet despair, it's Alfred, the stoic and distant hero, who stays by his side. A tale of tender ache and unspoken longing—an otherworldly BL romance, slow-burning and bittersweet.

(Source: Libre, translated)