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Yamaguchi (山口) is the director of S4's special programs. He later attends Yume's first solo concert.
Yamaguchi has short, flared brown hair giving him a youthful appearance. His deep blue eyes are slanted with a sharp corner.
He is very serious about is work and acts kind to those around him.
- He bears a resemblance to Shou from Pretty Rhythm Aurora Dream.
- He shares his seiyuu with Axel from OZMAFIA.