Gwendolyn "Gwen" Tennyson is the Human/Anodite cousin of Ben Tennyson.
- All, except Homesick Trilogy: Part 1, 10 Again, Artificial Consequence, and Ben 10: Invasion of the Omnitrix.
- The Sea's the Limit
- Dial Z for Zombified
- Ten Plus Twenty (only her alternate counterpart, Gwen 20)
- Happily Never Happened
- New Girl
- Man in the Stone Mirror
- Threat or Menace
- The Alternative (12 year old)
- Why Am I Here? (offscreen, but heard; 16 year old)
- Love Stuck
- Ultimate Solution (offscreen, but heard)
- Overcoming Adversaries
- Ledger Recon (killed)
- Ben 10: Alien Universe Unleashed (flashback to The Final Battle: Part 2; revived)
- Meet the Replacements
- Hooking Up With the K's
- Fate of the Helpers
- All episodes (main protagonist)
- There is a Gwen 10 that exists that found the Omnitrix when she was 10, and later when she was 15, put it back on after a break from it. This Gwen had experiences somewhat mirroring Ben 10's own.
- An alternate version of Gwen 10 appeared where Ben had come from an alternate timeline to tell her that he was the rightful bearer of the Omnitrix.
- In Gwen 10: Alien Force, an alternate Gwen appears, known as Gwen 10. She is the primary Gwen 10, and found the Omnitrix when she was 10 (not the same as in Gwen 10, as in her timeline, Ben didn't know about the Omnitrix or get the Omnitrix first), and now when she was 15. The end of the episode split this into its own timeline. (This episode became about an alternate counterpart of Gwen 10.)
- In Ten Plus Twenty, a Gwen known as Gwen 20 appeared. Her experiences mirrored events of Ben 10 as well as Super Ben. Some differences included Zennith having invented the Omnitrix (and still being together with Azmuth) as well as her trusting Gwen with Master Control. Charmcaster is her main opposition, as opposed to Ben and Albedo.
- A possible future was shown for Gwen 20 (in the same aforementioned episode) where her "golden years" of heroing were from age 15 to 26. At age 26, she became pregnant with her daughter Jennifer-Mercedes Tennyson. She was married to an unknown man who fought in the war against D-Soft. He lost and presumably perished in battle, or became one of D-Soft's brainwashed drones, with the latter being far less likely. Jen's middle name derives from Gwen's future best friend, Mercedes. Gwen properly delivered Jen at age 27, and went missing at age 39, while her daughter was 12. Jen raised herself with friends Mitch and Inferna. Gwen 20 visited this possible future with Super Ben.
The Alternative Gwen
- In The Alternative, an alternate version of Gwen was shown. She was aged 13 and the events of her life mirrored the standard Alien Universe one up until Vilgax came searching for the Omnitrix.
- Using several spells, she was able to be a major source of aid in getting Ben to have Vilgax relinquish the Omnitrix (regardless, the Omnitrix was destroyed in place of being properly retrieved, leaving that timeline's Ben with a remodel of Azmuth's Mechamorph armor "Aggression"). One of her spells unlocked Nigester for that timeline's 13 year old Ben's Omnitrix (albeit, the Omnitrix is destroyed now).
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