There's not much information available on Yamaguchi. Her singing career was very brief, lasting between 1980 and 1985 during which she released only three studio albums and one best-of, and as I'm writing this, none of these have been remastered on CD. Her most successful release was the third album "Tsukihime", which peaked at No.64 on Oricon weeklees and sold about 5000 copies. According to J-Wiki, she was labeled as a "synth utahime" due to how her elegant vocals combined with the synthesized sound. From mid-80's on, she withdrew to the background to compose songs for singers like Miki Imai, Junichi Inagaki, Masayuki Suzuki, and Yuki Saito. I found a site that lists some of those works. She had great potential as a singer though.