Comics Monica has a different story on a different timeline. She joined the Avengers after being given superpowers by a mysterious energy discharge in New Orleans harbor. She eventually rose to lead the team, working with Vision and Wanda in the process. She was widely respected by her teammates, and even after retiring from cape work because of an injury, she retained her powers and was frequently called back to duty. She also changed her name approximately a thousand times, from Captain Marvel to Pulsar to Photon, and landed on Spectrum, the name she currently uses.
So Why is Monica Rambeau on WandaVision?
Monica’s powers are the most likely nexus to the show. SWORD, the mysterious agency that we’ve seen monitoring Wanda’s neighborhood, rebranded when it made the jump from comics to television, from an agency monitoring for extraterrestrial threats to an agency monitoring “sentient weapons.”
One theory about what SWORD is doing has them monitoring beings whose powers are awakened by a zap from an Infinity Stone. Wanda’s powers were first awakened by the Mind Stone in Avengers: Age of Ultron, the same way Carol’s powers were granted by her blast from the Space Stone. Like the Carol of the movies, Monica got her powers from a mysterious energy blast, and she has a nexus to Carol in the MCU. It’s entirely possible Monica has powers she got from her association with Carol.
Monica can transform herself into electromagnetic energy. She can absorb energy, channel it into energy blasts, convert her body into any form of electromagnetic energy for travelling purposes, blind people with light flashes, fly through space unaided, read energy trails, and so on. She is by definition an sentient weapon, and chances are good she’s a SWORD agent on purpose because of those powers. Then again, she could just be working for SWORD and not have any powers. But that’s no fun.
In any case, we expect to learn much more about the MCU version of Geraldine on future episodes of WandaVision, and we’ll keep you posted as we do.