Gwendolyn "Gwen" Tennyson is a Human/Anodite hybrid Plumber, the paternal cousin of Ben Tennyson, and (along with Kevin Levin) was a front line member of his team during the first 3 shows. Gwen was first introduced when her parents sent her on a summer-long road trip with her Grandpa Max and her cousin Ben. She has, as Verdona refers to it, "The Spark".
In the original series, Gwen had short orange hair held by a blue hair clip and green eyes and wore a blue shirt with a cat logo on her chest and white capris. She wears a blue hairclip in her bangs. Her lucky girl outfit is worn in Lucky Girl and Tough Luck.
In the first and second seasons of Alien Force, Gwen wore a white shirt under a dark blue sweater, black stockings under a black mini skirt, and black high heels. Her hair has also grown longer and is now down to her waist. In the third season, she wears a red shirt paired with a black vest, gray pants, and black flats. Her hair is worn in a high ponytail. In her Anodite form, she looks like a mini version of Verdona. At the beach, Gwen wore a black bikini. Later on, she wore a white one. In Ultimate Alien, Gwen's outfit is similar to her outfit from the first two seasons of Alien Force, but with a red sweater while her hair still is now slicked back in a ponytail.
In Inspector 13, Gwen is seen wearing a violet tank with a white skirt and tennis shoes.
In Omniverse, Gwen still has her ponytail, but it's much shorter. She now wears clothes similar to her Alien Force seasons 1-2 outfit, except she now wear long white socks that go up to her knees with black mary janes. She now also has freckles.
Later on, she has shorter hair, which is now orange again, with her blue hairclip, a light blue T-shirt with her cat logo on it, a blue plaid miniskirt and wears blue-tinted glasses. In her new Anodite form, her skin turns purple, but her hair remains the same length, but is now made of mana and is pink and her eyes also turn pink. Her glasses also become a blue mask and she now wears blue gloves and her shirt is a darker blue with a light blue stripe. Her shirt cuts off and reveals her midriff and naval. She also wears blue boots, a dark blue miniskirt with a light blue stripe and a belt. The cat logo on her shirt also becomes black with pink eyes.
11 year old Gwen in Omniverse looks just like she did in the original series except that she has freckles and her cat logo is now bigger.
In the original series, Gwen is portrayed as an intelligent, mature but usually stubborn and smart-mouthed when it came to talking with Ben. Her quick thinking and good instincts give her an edge in dealing with dangerous situations. However, she's sometimes too trusting with people who want to exploit her goodwill, leading her (or others) into harm's way.
As she got older, she became even more mature, as well as becoming more caring and sympathetic. Her love for her cousin, Ben, became more clear in the future series as well.
Gwen is willing to forgive villains for their wrongdoings, the most notable being Kevin, if she believes they've changed for the better. Although her intelligence and knowledge have only increased since the original series, she's not completely free of her naivete, and can be arrogant at times as well.
In the original series, Gwen went on a road trip with Grandpa Max and her cousin Ben. She and Ben constantly argued (like about Ben misusing the watch or about Gwen's powers) however they were shown to deeply care for each other. Gwen also shows deep affection for her Grandpa.
Gwen also uses newspapers and her laptop to research any strange people and creatures they might come across. She is a capable gymnast and has had some training in martial arts.
In Ben 10,000, her future self says she got her black belt years ago and it's come in handy many times. She has also shown an almost innate skill in using various complicated devices or objects, such as in the "What If?" episode Gwen 10, when Gwen gained the Omnitrix instead of Ben and proved to be far more adept in its use than he initially was, even being able to write her name with fire while Ben took a month to be able to do it.
She is depicted as snobbish due to her young girl attitude. Overall, her most notable skill is her innate, if latent, ability at magic. Having within her what Charmcaster calls a "magical aura", Gwen is capable of using various magical artifacts, charms, talismans, and spells. When using the magical, luck-giving Charm of Bezel, she was able to activate it instinctively, whereas Hex did or could not. Later on, when she takes and uses his staff, she was able to make it fire a yellow-colored energy blast (according to Charmcaster), only someone of Hex's skill should have had that ability.
Gwen starts tapping into magical powers when she procures a small book of spells from Charmcaster in the episode A Change of Face. On two occasions, Gwen uses the Charms of Bezel's incredible and unique powers and abilities to become a superhero by the name of "Lucky Girl". At first, her ability drew from the luck-granting, Charms of Bezel, which innately gave her perfect luck and the abilities and powers of probability manipulation. While wearing the charm, any action she took would result in a Rube Goldberg-style chain reaction benefiting her original intent, but usually injuring Ben in the process. She later brought back her superhero identity after obtaining the legendary Keystone of Bezel, which greatly strengthened and increased her natural abilities to super human levels rather than just affecting her luck. Both times, Gwen was forced to retire when the Charms of Bezel were destroyed. Gwen has also appeared to have two swimsuits; one which appeared in Divided We Stand and the other appeared in A Small Problem.
In Perfect Day, a superhero form was given to her by Ben as a form of petty torment while they were inside Ben's dream battling his imaginary bullies. In this form, she only exhibited the power of enhanced biting. Ben called her Ultra Rat.
Gwen forms a rivalry with Charmcaster during the series.
Five years prior to Omniverse
In Ben Again, Gwen is intrigued by the sudden increase in maturity of the 11 year old Ben whose mind has been switched with 16 year old Ben. Eventually she helps to defeat Eon and her memory is erased about the events of the episode except for a thing that 16 year old Ben in 11 year old Ben's body told her in a vague memory. In Evil's Encore and From Hedorium to Eternity she helps to defeat Dr. Animo and Zs'Skayr respectively. She appears in flashbacks in other episodes of Ben 10: Omniverse.
Powers and Abilities
Mana Manipulation and Energy Control
Gwen's magical powers are revealed to be of alien descent inherited from her grandmother Verdona, a member of a free-spirited alien race of pure energy beings known as Anodites, from the planet Anodyne. Verdona tells Gwen that, like herself, Gwen is also an Anodite; she has "the spark" and thus is able to freely manipulate and control mana. 
Gwen can create energy blasts, shields and barriers. Her powers are mainly used like energy constructs, allowing her to make many different types of structures like, hammers, protective helmets, battering rams, beams to grab and throw enemies, stepping stones/platforms to carry her and others though air, safety nets; it also turns out Gwen can also synchronize her martial arts styles with her energy manipulation abilities.
Gwen has more mana-related powers: She can find any person she wants by detecting the traces of mana that he/she left. She can also absorb mana.
She also has the abilities of telekinesis, teleportation, heat vision, telepathy and healing living beings. She can also transform into an Anodite. In her Anodite form her powers are greatly enhanced, having demonstrated the abilities of flight and size alteration.
Magic and Spells
Gwen is seen using magic through casting magical, mystical spells, and reciting spoken incantations thus enabling her to use various magical powers and abilities.
On two occasions, Gwen used the five mystical Charms of Bezel to become a super heroine by the name of Lucky Girl. At first her power and ability drew from the luck-granting Charm of Bezel, which innately gave her complete and perfect luck in everything she did. While wearing the charm, any action she took would result in a chain reaction benefiting her original intent, but usually injuring Ben in the process. She later brought back her superhero identity in the episode Tough Luck after obtaining the legendary lost Keystone of Bezel, which greatly increased her natural abilities to superhuman levels rather than just affecting her luck. Both times, Gwen was forced to retire when the five Charms and Keystone of Bezel were destroyed.
Gwen claims that being self-taught in magic is not so easy as it looks. Trying to get into Yahwahtacsip she mistakes its name for "Wah Di Tah", saying that she knows how to read it but not pronounce it correctly.
Later Gwen begins to cast spells and recite spoken incantations again. Gwen is less knowledgeable than Charmcaster and Hex, having been self-taught, but she is also more powerful than Charmcaster until Charmcaster obtained the Alpha Rune, after which she was powerful enough to defeat Gwen with ease.
She has also learned to astral project through magic.
Athletics and Martial Arts
Gwen is a capable gymnast and martial artist. Even in the original series, she was occasionally shown to be quite good in hand-to-hand combat fights against enemies who had normal strength levels. She was part of her school's Jujutsu team. At the age of fifteen it is revealed that she has already received a black belt in Taekwondo.
When preferring to use her athletics during fights or when she cannot rely on her magical and Anodite powers, she is shown to be highly skilled in hand-to-hand combat.
According to Dwayne McDuffie on one of his forums, Gwen does martial arts, horseback riding, softball, skiing and cross country. Turns out that Gwen can also synchronize her martial arts styles with her mana manipulating abilities. Gwen is shown creating a whip of mana energy and using it as a weapon along with her fighting skills. When Kevin asks her how she learned it, she states remembering it from an old movie.
Knowledge and Intelligence
Gwen is shown to be very intelligent and knowledgeable. Before discovering her dormant Anodite powers, she would often use her superior intellect to help Ben solve problems. She is shown to be a bit of computer and tech wiz as well. She has her own laptop which she uses to research enemies she meets.
In Back with a Vengeance, Gwen switched the Omnitrix with a bomb, saving both Ben and the world, showing her cleverness.
- All episodes except Gwen 10 and Goodbye and Good Riddance.
- Ben 10: Secret of the Omnitrix
- Ben 10: Destroy All Aliens
Ben 10 Shorts
- Snack Break
- Survival Skills
- Radio Dazed
- Sleepaway Camper
- Dogged Pursuit
- Let the Games Begin
- Handle with Care
- Road Trip Rumble
Ben 10: Alien Force
Ben 10: Ultimate Alien
Ben 10: Omniverse
- The More Things Change: Part 1 (16 years old)
- A Jolt from the Past (11 years old)
- Trouble Helix (11 years old)
- It Was Them (11 years old)
- Of Predators and Prey: Part 2 (11 years old)
- Many Happy Returns (16 years old)
- Malefactor (11 years old)
- Ben Again (11 and 16 years old)
- Showdown: Part 1 (11 years old)
- Showdown: Part 2 (11 and 16 years old)
- The Frogs of War: Part 2 (16 years old)
- Evil's Encore (11 years old)
- Mud Is Thicker Than Water (11 and 16 year old)
- For a Few Brains More (11 and 16 years old)
- Max's Monster (11 years old)
- Mystery, Incorporeal (16 years old)
- Weapon XI: Part 1 (16 years old)
- Weapon XI: Part 2 (11 and 16 years old)
- Charm School (16 years old)
- From Hedorium to Eternity (11 years old)
- Third Time's a Charm (16 years old)
- The Most Dangerous Game Show (16 years old; cameo)
Cartoon Network Action Packs
Gwen has appeared in every action pack, except for Old Friends, Plumbers' Kids, It's a Set Up and twoSome.
- Ben's Knightmare
- War of the Worlds
- Triple Threat
- Double Negative
- The Dark of Knight
- Galaxy Wars
- Science Friction
Ben 10: Omniverse
- Ben 10 (Issue 1, Issue 2, Issue 3, Issue 4)
- Ghost Ship
- Parallel Paradox
- Doom Dimension
- Wanted: Kevin Levin
- In Ben 10: Alien Force Gwen can shoot bolts, grab enemies and enable auto defense mode as well as float around for a small amount of time.
- Ben 10: Protector of Earth (Playable only on Nintendo DS version, with a code) (Skin)
- Ben 10: Alien Force (Playable only on the Nintendo Wii, Sony PS2, and Sony PSP versions on levels 3 and 8) (Regular Character)
- Ben 10 Alien Force: Vilgax Attacks (Non-player character) (Regular Character)
- Ben 10 Alien Force: The Rise of Hex (only for Wii and Xbox 360) (Regular Character)
- Ben 10 Ultimate Alien: Cosmic Destruction (Non-player character) (Regular Character)
- Punch Time Explosion and XL (Assist Character)
- According to Ben, and proven by Gwen, Gwen's favorite flowers are yellow roses.
- According to Rook, Gwen's birthday is in December.
- Also, Gwen and Ben were born on the same day.
- According to Diagon, Gwen is the most powerful being in her dimension.
- Gwen is Verdona's favorite grandchild.
- In the initial stages of the original series' development, Gwen was written as a classmate who happened to be friends with Ben, but otherwise had no familial relation to him.
- Gwen isn't too fond of Upchuck's powers. It is also shown on several occasions that she isn't fond of Goop.
- Gwen's DNA was sampled and randomized into the Unitrix to create Eunice.
- Gwen's new look in Omniverse bears some resemblance to Velma Dinkley from the Scooby-Doo! franchise.
- Gwen made an appearance in the fifth episode of the sixth season of Robot Chicken, Hurtled from a Helicopter into a Speeding Train.
- According to Matt Wayne, Gwen has much more control over her mana powers than shown.