Gabriella "Gaby" Fernandez (portrayed by Mayteana Morales, seasons 1 & 2; Melissa Gonzalez, season 3) is Alex's younger sister. She is also the fourth youngster to whom Ghostwriter appeared. While her teammates go to Hurston, Gaby goes to Washington Elementary School.
Gaby and her older brother Alex lived with their parents in the family's corner store, or bodega. She and Alex originally shared a big bedroom in the back. It had a dividing curtain in the middle for privacy. Then at the end of the second season, her brother and her dad build Alex his own bedroom in one of the spare storage rooms. Then in the third season, she had the old bedroom all to herself now.
In episode one, Gaby's backpack was stolen by a masked thief while she and her brother Alex were walking to school. She later revealed that backpacks were stolen from other students at her school. Alex, Jamal and Lenni were already working the "case" to identify the thieves. However, as Alex pointed out, they had to work in secret to avoid any inquisitions from Gaby about their activities. When Ghostwriter revealed himself to Gaby, the team's activities were, in a way, exposed. So, Jamal Jenkins, Lenni Frazier and Alex had to "explain things to Gaby."
As the show progressed, a few key traits about Gaby were revealed:
- Trivia queen
- "Best speller in her class," as Gaby herself put it
- Enjoyed solving word (and other) puzzles
- Somewhat talkative
- Very sensitive and yet strong willed
At one time, a strong desire outshined Gaby's better judgment. During the "Am I Blue?" story arc, Gaby stole $50 from her family's cash register to purchase a Galaxy Girl costume, with the hopes of returning it for a refund. However, she couldn't return it and eventually most of the clues from that case pointed to her. Gaby was eventually found out about stealing from the bodega register, but to make matters worse, Lieutenant McQuade questioned Gaby about the theft of a model spaceship, the QTC 5000, that was supposed to be used for a Galaxy Girl event at Hurston Middle School. She even began have bad dreams where she was Galaxy Girl and a Makva that resembled Lt. McQuade continuously accused her to her face about her crimes. However, also in her dreams, were her friends. They assured that she was still one of them and that everybody does bad things sometimes. Furthermore, everyone deserves a second chance. Gaby, in the person of her favorite heroine, hugged all of her friends for their helpful words. There was also a stranger wearing mask over their face whom Gaby hugged last. This person began to reveal themselves to Gaby when the dream cut off...all Gaby could say was "Ghostwriter!" Gaby apologized for stealing from her family, but she was still grounded for a week. Meanwhile, the real culprits behind the "Cutesie" ship theft—the creators of "Galaxy Girl," twins Abigail and Jordan Jefferson—were exposed.
During another outing, Gaby and Alex were at odds with Tina and Lenni over a misunderstanding following an accident between Mr. Fernandez and Max Frazier, Lenni's dad. Ghostwriter nearly faded from existence as a result of the in-fighting among the four members, and after Rob, Jamal and GW formed a letter to the team reminding them of some of their past adventures together, the fights and disagreements ended. As for Ghostwriter, the team was the "link" that sustained him. With them back together, GW could remain to help them solve mysteries another day.
- "Ghost Story"--GW reveals himself to Gaby, who lets out a shriek of terror, alarming Alex and the others. The ghost writes, "Gaby thirsts for knowledge." Afterwards, Alex, Lenni and Jamal sit Gaby down to explain who or what the "thing" is that is writing to her.
- "Over a Barrel"--Gaby gets sick while working in a community garden. The team moves to find out what is causing people to become ill, as well as what it is that is killing the animals that reside in the garden.
- "Am I Blue?"--Gaby wants a Galaxy Girl costume, but does not have the money for it. So, she decides, unfortunately, to steal money from the bodega cash register and is eventually caught. But when an expensive model ship used for stage shows is stolen, and later finds its way into Gaby's trunk, she soon realizes that she is truly between a rock and a hard place.
- "Get the Message"--Gaby, Alex, Tina and Lenni are bickering with each other over an accident involving Mr. Frazier and Mr. Fernandez and the newspaper article statement given by Tina the day after. What's more, Ghostwriter is feeling the effects of the rift and is fading. If Rob and Jamal don't act quickly, the ghost may disappear forever!
- "Four Days of the Cockatoo"--Gaby finds a rare palm cockatoo in a back alley dumpster, and soon becomes the prey for a couple of bird thieves who are also accomplished masters of disguise.
- Gaby is in the epilogue of the 2019 reboot Ghostwriter book Trinity, along with Jamal.