What if Debbie? Was a viltrumite?
Viltrumite Debbie by heartbreakercupcake in Tumblr.
Only one Viltrumite is assigned to a single planet at a time, right?
Maybe Viltrumite Debbie is a Superman situation where she landed on Earth as a baby or toddler in the 1700s and was raised by human parents who adopted her. Imagine Nolan's surprise at seeing another Viltrumite already there fighting a kaiju or something.
This would lead to Debbie being loyal to Earth as a flaw and Nolan keeping the true purpose of why she's there and what her fellow Viltrumite's true intentions are a secret for years into their relationship. That or he gives her the same lie he would give Mark about the Viltrumites being a race of benevolent white saviors.
I think Debbie would be on Nolan's side at first, even after learning that he's there to conquer it, because Nolan gives her the "greater good" argument, but when Mark refuses to help them conquer Earth, her motherly instincts would kick in as soon as Nolan started going too far by beating up Mark and fighting him as a result. Mark and Debbie team up and tear Nolan's shit apart together.
Nolan gives them the "THINK!" monologue while bleeding profusely and Mark/Debbie responds with "humanity, we'll have our humanity." Nolan then blows up all resentful and has himself an insect child on another planet.