Perhaps bring back Sherry Birkin?
She was supposed to be in Resident Evil 5, but was cut out for some reason. She could return in Resident Evil 6.
Another character that crossed my mind was Steve Burnside, the most annoying supporting character in the Resident Evil series. He died at the end of Code: Veronica X, but he was injected with the T-Veronica virus and his body was retrieved by Wesker, who said that Steve could be revived.
Steve was a rather unpopular character due to his attitude at the beginning of Code: Veronica X, but he was only 17 at the time, so mature adult versions of Steve and Sherry could return as partners in Resident Evil 6. It also helps that both characters are linked to Claire, which could open up various possible plots.
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