|Role||Yumi's younger brother|
|English voice actor(s)||Barbara Weber-Scaff|
Hiroki Ishiyama is Yumi's younger brother and a sixth grader at Kadic since season 3. Although Yumi is usually annoyed by Hiroki, she still loves him, as shown on numerous occasions. He is the son of Mr. and Mrs. Ishiyama. Like his parents, he is a recurring character in the show.
Hiroki is usually portayed as a typical bratty brother. He is portayed to be a somewhat mean to Yumi, but in a brotherly kind of way. He will usually help Yumi in exchange for her doing him favors, such as doing his homework for him. He loves to ask Yumi "if she is dating ulrich?" He is shown to deeply care about Yumi when she is in danger. He is shown to be maddly in love with Milly, but does not know how to act on these feelings.
- Hiroki first appears in Exploration , in this episode he appears as a bratty little brother. Hiroki is present at the family's game night and is playing a video game on what appears to be a Nintendo DS.
- Hiroki's next appearance is in The Chips Are Down where he is at the table when his father says that he was unable to find work at the temp agency. He is later shown to be present when the Ishiyamas's win the lottery, through Ulrich's cheating.
- In A Bad Turn, Hiroki call's Yumi to ask her to save him and his family from the big monster. Hiroki watches as his father and later Ulrich defends against the monster's attacks. When Ulrich cut's off one of the monster's legs Hiroki runs towards him and the monster. This causes Ulrich to get knocked unconcious by the monster.
- Hiroki is mentioned in evolution but never seen.
- Hiroki is an excellent lock pick,
- Judging by how much he plays video games, he seems to be a decent gamer.
- Hiroki is a terrible listener.
- Hiroki is great with dogs.
- Hiroki made his first appearance in Season 2 episode, Exploration. He never seen nor mentioned in Season 1.
- In A Space Oddity, Hiroki call Kiwi with "Leechie".
- 'Hiroki' is a Japanese word for "Abundant joy/strength".