Sôsuke Ajino achieved global renown as a young pianist, performing in major concert halls. A custom grand piano with exceptionally heavy keys was designed specifically to accommodate his powerful playing style. His career shattered abruptly in a severe car accident that caused permanent damage to his left hand and claimed the life of his fiancée. Plunged into despair, he abandoned performing and sold his custom piano, which eventually ended up abandoned in the Morinohata district's forest.

Initially teaching at a music university post-accident, Ajino struggled to find students whose interpretations engaged him. After a decade, he resigned to search for his former piano, only to discover it decayed and unplayable within the forest. Unable to leave the area, he accepted a position teaching music at Moriwaki Elementary School. His stern demeanor earned him the nicknames "Ajino the Deathbringer" and "Funeral Ajino" among students. He maintained a detached teaching approach until encountering Kai Ichinose, a student possessing the natural ability to play the derelict forest piano.

Recognizing Kai's innate talent, Ajino became his dedicated mentor and surrogate father. He guided Kai's technical development, preparing him rigorously for the International Chopin Piano Competition in Warsaw. This mentorship marked a profound shift for Ajino, moving him from disillusionment to renewed purpose. He emphasized artistic freedom over competition rankings, viewing Kai's growth as a path to transcend his disadvantaged background. Ajino's coaching extended beyond technique, focusing intensely on helping Kai develop his unique musical voice and emotional expression. His influence also indirectly affected other pianists, such as Wei Pang, whose style notably resembled Ajino's pre-accident playing.

Ajino possesses medium-length black hair, black eyes, and typically wears formal attire: a white shirt, green trousers, a green tie, and a black jacket. His background includes an unnamed mother.