Gwendolyn Catherine "Gwen" Tennyson is Ben's 10-year-old first cousin. Also, she dates Kevin Levin.
In the original series, Gwen shares a love-hate family relationship with Ben. Before discovering her dormant magical and Anodite powers, she would help Ben by simply using her innate cleverness and intelligence. She is known to be a capable gymnast and martial artist and has received a black belt in taekwondo. Gwen is a very good student, especially in the episode "It's Not Easy Being Gwen", which shows that Gwen takes her education very seriously and in Ben 10: Omniverse, she is shown leaving for Ivy League College.
Overall, her most notable skill is her incredibly strong magical abilities. Having within her what Charmcaster calls a magical aura (which is bright blue in the original series but eventually changes to bright shades of pink, magenta, fuchsia, and varying shades of violet in the two sequels), Gwen is capable of using various magical artefacts, charms, talismans, spellbooks, and spells. On two occasions, Gwen has used the five Charms of Bezel to become a superheroine by the name of Lucky Girl. At first, her power and ability drew from the luck-granting Charm of Bezel, which innately gave her perfect luck. 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 superheroine identity after obtaining the Keystone of Bezel, which greatly increased and strengthened her natural abilities to superhuman levels rather than just affecting her luck, and which granted her superhuman agility, stamina, and endurance. She was later forced to retire when the Charms of Bezel were again destroyed. However, in a twenty-year alternate future timeline, thirty-year-old Gwen had gone to the mystical interdimensional world of Bezel to receive a brand new set of mystical charms.
Five years later, in Ben 10: Alien Force, her magical abilities have greatly strengthened and she is easily able to control and manipulate pink/magenta/fuchsia-colored glowing energy from her magical aura to create energy blasts, shields, barriers, ropes, chains, stepping stones, and other weapons and items. Her paternal grandmother, Verdona, is an vastly powerful Anodite alien from the planet Anodyne; a free-spirited alien species who possess the ability to manipulate and control a supernatural force-like substance called manna (also referred to as life energy, chakra, or chi) which is the very lifeblood essence of all magic and life itself. Gwen is also able to take on an Anodite energy form just like her grandmother whenever she gets very angry, enraged or stressed. In the mystical world, Ledgerdomain, Gwen's magical and Anodite powers are tremendously supercharged and magnified.