Hiroki

[The Hornet]

http://tvtropes.org/pmwiki/pmwiki.php/WMG/CodeLyoko
"Think about it. He's nowhere to be seen in all of season 1, even on occasions where he really should have been, such as in "Holiday in the Fog" when Yumi's family were going on vacation. This is because Hiroki isn't Yumi's biological brother, but instead he was adopted by the Ishiyama family sometime between "False Start" and the beginning of season 2."