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Hiroki Ishiyama is Yumi's eight-year-old younger brother, a 2nd grader 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 lose focus on the monster and get knocked unconcious by the Krab. 

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 greeted Yumi at the Cafeteria and introduced her to Johnny and told her to be nice for him or he will snich on her.

He also appeared in False Lead, where he got Odd's Tamaguchi after the others took it from him as he was too attached to the device. Hiroki got quickly bored with Tamaguchi and threw it onto the street where it got destroyed.

In Final Round, Yumi was babysitting Hiroki but she needed to run to the Factory to protect Lyoko's Core. Unfortunately, Hiroki didn't want to let her go just because he could do it. He threatened her to call her parents if she left the house and Yumi said to him that their father is calling her so she gave him her phone. It turned out that Jeremie was on the line but before he could say that to Yumi she already left the house.

Season 4

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

In the episode Hot Shower, he participates along Johnny and his classmates in the physics class with Yumi and her class to build a model of solar system. He and Johnny were given a task to make models of Earth and Moon but they weren't taking their job seriously much to Yumi's frustration. When the meteorite was about to hit the school, the principal, thanks to Mrs Hertz and Yumi persuasion decided to make an evacuation. However, Hiroki and Johnny saw Yumi sneaking to the science building so they decided to follow her. Hiroki demands her to tell her what she's up to. They initially don't belive her about the meteorite but they quickly change their mind. Hiroki helps Yumi to unlock the roof door showing he's a great locksmith.

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.

In the episode Canine Conundrum, he goes to Penchak Silat training with Johnny, Sissi, Yumi and Ulrich. When Milly and Taimya appear in the gym as well to make the interview it turns out that he invited them. Later when Kiwi 2 clones attacked the gym he helped the others to fight with them.

He also appears in A Space Oddity and he takes care of Kiwi on Yumi's request. At first he finds having a dog at home as a great fun but he quickly gets tired of him.

In the episode Music to Soothe the Savage Beast Hiroki is shown to have some spare tickets for Subdigitals concert but he doesn't want to sell them to Odd in lower price as he claims that other people are willing to pay more for them.


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 Lost at Sea and 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".


  • Despite his relation to Yumi, Hiroki is neither seen nor mentioned for the entire first season. Sophie Decroisette confirmed in an interview that Hiroki was added at the request of producers for Season 2, as he's not mentioned in the show's original graphic bible.[1][2] She theorized an in-universe explanation that he was away at another boarding school at the time.
  • In A Space Oddity, Hiroki called Kiwi "Lychee".
  • 'Hiroki' is a Japanese word for "Abundant joy/strength".
  • He does not make any appearances in Code Lyoko Evolution, but he is mentioned by Yumi in False Pretenses.
  • In the Finnish dub, he was voiced by Hanna-Maija Nikander.
  • In the Polish dub, he was voiced by Elżbieta Kopcińska-Bednarek.



  1. "Interview - Sophie Decroisette, partie 2". Ces Dessins Animés-Là qui méritent qu'on s'en souvienne. March 11, 2014.
  2. "The Graphic Bible of Code Lyoko" -