Takeho Ishiyama (also spelled Takeo) is Yumi's and Hiroki's father. He is a working man and the provider for the Ishiyama family.
In Laughing Fit, he and his wife, Akiko Ishiyama, are shown fighting and wouldn't talk to each other, with him going as far as to move in with a friend. In The Chips Are Down, he is also shown to have lost his job and is forced to return to Kyoto, Japan, if he can't find a new one, but in the end of the episode he got a new job and he and his family were able to stay in France.
He often worries about his daughter who sneaks off at night to get to the factory. He also seems to love dogs, as shown when he meets Kiwi, despite having previously stated that he hates all domestic animals.
In the English dub of Code Lyoko, he was voiced by David Gasman. In the French version of Code Lyoko, his voice was either provided by Mathieu Moreau or Frederic Meaux. There is contradictory information about this.