User blog comment:PhotonCommander10/Ben 10: Ultimatrix Forever/@comment-5905231-20101219005601

Gwen just disappeared but was brought back when Sparky and Abaddon's attacks hit each other. They wonder why but in the season finale they find out Abaddon used a device to put her in the Null void and wiped everyone's memories. Ben, Max, Professor Paradox and Abaddon remembered because Paradox was in a different time when it happened and briefly took Ben and Max to a different time to save them.