Shirou Nishi, grandfather of Seiji Amasawa, runs a small Tokyo antique shop near Shizuku Tsukishima's home. A skilled violin craftsman, he teaches violin making and maintains his own workshop alongside the antique business. His shop houses unique items, including a statuette of a formally dressed cat named Baron Humbert von Gikkingen, which deeply inspires Shizuku's creativity. He also possesses a restored grandfather clock depicting the King of the Dwarves and the Fairy Queen, sharing its story during Shizuku's visits.

Nishi displays a kind, gentle nature, acting as a mentor figure, especially to Shizuku. He encourages her artistic pursuits and imparts wisdom about creation. When Shizuku wishes to write a story featuring the Baron, Nishi grants permission, requesting to be the first reader of her finished work. Later, he provides constructive feedback on her manuscript, recognizing its raw potential while stressing the need for further refinement. He illustrates artistic development with a geode metaphor, comparing unpolished talent to natural stones containing hidden gems that require diligent polishing to reveal their true value.

His supportive guidance extends to his grandson Seiji, backing his ambition to become a master luthier. Nishi stands by Seiji's decision to study violin making in Cremona, Italy, despite family opposition, reflecting his belief in nurturing passions through dedication and hard work. Nishi also demonstrates musical talent, joining impromptu performances, such as accompanying Shizuku and Seiji's rendition of "Take Me Home, Country Roads" with his instrument.

His interactions with Shizuku's family further underscore his stabilizing influence. During a dinner conversation, he reinforces balancing creative aspirations with practical responsibilities, echoing concerns voiced by Shizuku's parents about her academic commitments. His presence consistently offers reassurance and perspective to those navigating personal and artistic challenges.