She features in Back in Action: Alien Universe.
In her debut in New Girl, she is depicted as a "blonde dimply country girl, dressed in blue overalls and plaid red and white underneath."
Later, in Hooking Up With the K's, to match her more closed-off nature following brainwashing by the Thep Khufan Armada, she has completely transformed to sporting all black, from her hair to sweater and pants.
Before her brainwashing, Mercedes displayed an overly enthusiastic, extroverted nature combined with childlike curiosity. Tightly-knit with them, she was swift to tell about discovering the Alphaline Omnitrix in her backyard.
- Main article: Mistress#Mercedes Barnes
After moving with her family to Bellwood before Hooking Up With the K's, she has shown lingering effects of her transformation into the infamous Mistress from her time with the Thep Khufan Armada - being cold and rather ambivalent with the mention of Scarogus.
Unlike most, she wasn't initially fascinated by Ben's heroism, though with time and proximity to him, she seems to be at the very least interested in Ben as a person.
She leans closer to ambiversion or introversion now as well. In Madame Regle, Gwen seemed to have tapped into Mercedes' memories of being Mistress. Though the former has sworn to secrecy, Mercedes' reluctance to share is likely in fear of losing Ben.
She notes that she isn't as close with her family following the effects of her time as Mistress.
- In New Girl, she finds Azmuth's latest invention. After attempting to figure out what it is, half of her home explodes with her in the thick of things.
- In Man in the Stone Mirror, during Endgame, she is mentioned as having gone missing despite the events of the explosion being revealed.
Back in Action: Alien Universe
- New Girl (first appearance)
- Man in the Stone Mirror (mentioned only in Endgame)
- Ultimate Solution (Endgame only)
- Overcoming Adversaries (captured by Loitus)
- Ledger Recon (as Mistress; brainwashed)
- Welcome to Chronospect (as Mistress; brainwashed)
- Hooking Up With the K's
- The Armaggedroid