Dr. Romero is a scientist, an ex-associate of Donnagon Giggles and an ally of the Cortez family.
Prior to the events of the film, Romero moved to what became an invisible island (due to the transmooker device) and created miniature animals and to go with them, mixed varieties of mini animals. Things went wrong when Romero applied a growth serum to his little mixed creatures and ended up hiding in his house. He was afraid that they would eat him up until when he tried to stop Donnagon that the creatures liked Romero.
Spy Kids 2: The Island of Lost Dreams
Spy Kids 3: Game Over
At the beginning of the film, Romero sent Juni some pets: Miniature Sharks. Also, he gave warning in his letter that Juni shouldn't poke his finger in the bowl, as the sharks are still man eaters.
When Carmen and Juni summoned all their allies to help fight against the Toymaker's video game robot gorillas, Romero came riding on a spork. Carmen gave him a pair of 3-D glasses to see the enemy, but they're a little broken. To Romero, they're perfect. He then got on the spork and charged at one of the robot gorillas. After the battle, Romero joined the Cortezs' extended family.