Chang Lu Steiner is the twin sister of Chang Gai Steiner and his second-year classmate at Seisho Academy. Academically gifted and athletically talented, she particularly excels at soccer. Despite her outward strength, she internally relies heavily on her brother and consistently accompanies him. She shares Gai's frustration with Daisuke Dojima's overprotective behavior, maintaining her distance from him. Unlike other Shibuya Defense Service members, she does not recall the childhood prophecy given by Milo.

Physically, she has light brown hair and green eyes. Her standard attire is the Seisho Academy uniform: a green skirt with brown details, a ribbon, and a brown coat. When piloting her String Puppet mecha, she wears a specialized yellow and orange suit.

As part of the Shibuya Defense Service, Chang Lu pilots a String Puppet to combat the mechanical Revisions after Shibuya is transported 300 years into the future. Her synchronization with the mecha is immediate due to its design for her capabilities. She participates in defensive operations alongside her brother, Daisuke Dojima, Marin Temari, and Keisaku Asano. Her combat role emphasizes coordination with Gai, reflecting their interdependent relationship.

Following Gai's death in Season 2, where Daisuke kills him in an act of revenge, her specific reactions or subsequent development are not detailed. Throughout the documented events, her narrative arc remains closely tied to Gai, with no further expansion in spin-offs, movies, or other media.