Isshun de Chiryou Shiteita no ni Yakutatazu to Tsuihou Sareta Tensai Chiyushi, Yami Healer Toshite Tanoshiku Ikiru

Born a poor man in the slums, Zenos never had opportunities in life, so he decides to forge his own by teaching himself the curative arts after a chance encounter with a healer. When an adventuring party led by a man named Aston extends him an invitation, Zenos is elated—only to then be treated like dirt by his teammates. Despite Zenos's diligent efforts to hone his skills to nothing short of perfection, the party deems him useless and kicks him out. Bereft of money and options, Zenos starts an illegal clinic and becomes a shadow healer. Word of his healing genius spreads quickly throughout the city's underbelly, and as his reputation grows among the dregs of society, knowledge of Zenos's esteem reaches the wrong ears... (Source: J-Novel Club)