Here is Karen from Code Geass and Akiza from Yugioh.
Why do these two have the exact same design? Is there any background story about the character designer?
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Sign up to join this communityThere's nothing really beyond the similar appearance which which isn't a new thing as various series have done this, One Piece & Fairy Tail, Parasyte & Yuri!!! On Ice, Hunter X Hunter & One Punch Man (Source)
neither the Yu-Gi-Oh Wikia on Akiza (Last Dot Point) or The Code Geass Wikia on Kallen (7th Dot Point) says anything beyond how similar they are
Akiza's
Akiza bears a striking resemblance to Kallen Kōzuki from Code Geass both sport a crimson theme as well as a similar hairstyle. Personality-wise they are not exact, but similar since they try to isolate themselves from the present society during the story and are hostile to the people they perceive as enemies. They also have possible romantic feelings for the main protagonists of their respective series.
Kallen's
Aki Izayoi from Yu-Gi-Oh! 5D's bears a striking resemblance to Kallen; both sport a crimson theme as well as a similar hairstyle. Personality-wise they are not exact, but similar since they try to isolate themselves from the present society during the story and are hostile to the people they perceive as enemies. They also have romantic feelings for the main protagonists of their respective series.
If one was inspired by the other, it would be that Akiza was inspired from Kallen as
while
but without any further information, i'd say it's just coincidence
Because nothing under the sun is original, they're just unique in their own ways.
It was probably either a coincidence or an inspiration.