Seiji enters the world in the early 21st century, a period defined by significant societal challenges stemming from a declining birth rate. By his adulthood, robots outnumber humans globally. A professional pianist, his career as a concert performer ends abruptly three years before the main events due to a severe accident that leaves him unable to play at that level.

He now lives with Yuna, an android model identical to the nurse android that cared for him during his hospital recovery. Despite the prevailing societal view of androids as mere machines, Seiji treats Yuna with dignity and humanity. Seeking to reconnect with music, he decides to teach Yuna piano. His instruction proves successful, as Yuna develops considerable piano skills under his guidance.

Seiji's dedication extends to taking Yuna to experience a live piano concert. This exposure advances Yuna's playing considerably, elevating her performance level further.