Hiroki Ishiyama is Yumi's nine-year-old younger brother and a fellow student at Kadic Academy. Although Yumi is usually annoyed by Hiroki, she still loves him, as shown on numerous occasions. He is the only son of Mr. and Mrs. Ishiyama.


Hiroki is usually portayed as a typical bratty brother. He is portrayed 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 madly in love with Milly, but does not know how to act on these feelings. 

Hiroki frequently tries to get Yumi and Ulrich to go out with each other, and has openly told Johnny that he has no hope with Yumi.


Season 2

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 makes a brief appearance in The Key, answering the door for Ulrich, and asking Yumi if she is dating Ulrich yet. 

Season 3

Hiroki started attending Kadic Academy in Straight to Heart, having skipped a grade.

He also appeared in False Lead, letting Odd's Tamaguchi get destroyed.

Season 4

He appears again in Skidbladnir and does Yumi a favor so they can free Aelita and Jeremie.

In the episode Lost at Sea, Hiroki steals Yumi's diary and Ulrich finds it. To help Hiroki apologize to his sister, Ulrich secretly returns the diary, but Yumi thinks it was Hiroki and forgives him.

He also appears in A Space Oddity and he takes care of Kiwi.


Season Two

Season Three

Season Four

Appearances in other Media



His best friend is Johnny, who has a crush on Yumi. He and Johnny are both highly capable locksmiths, as revealed in Hot Shower.

He has a crush on Milly, which became a minor plot point in Canine Conundrum.


  • Hiroki is an excellent lock picker. 
  • Judging by how much he plays video games, he seems to be a decent gamer. He owns a Gameboy Advance SP.
  • Hiroki is a terrible listener.
  • Hiroki is great with dogs.
  • He knows Morse code, as does Yumi.


Hiroki is a romanized Japanese given name with several possible meanings depending on how it is written. Ishiyama is the romanization of the Japanese family name 石山, which means "stone mountain".