He was the main antagonist of the second half of Season 2, after Big Hero 6 were declared figutives for defending Globby, who was falsely accused due to Nega-Globby taking control of him. However, after the season 2 finale, "Legacies", Cruz became an ally of the team instead.
Diego Cruz used to live in San Fransokyo years ago; during his childhood, he was a friend of Cass and both visited Joe's Diner frequently. One night, Cruz and his father went to watch a film, but on their way home, a villain attacked them. Cruz's father was able to save his son but was killed while trying to protect him. Boss Awesome showed up at the scene too late, and because of this, Diego developed a hatred toward superheroes.
Cruz later moved away from the city (presumably because of his father's death) but years later he moved back with his daughter Megan. Cruz soon enlisted into the San Fransokyo police force as the newest chief. However, this increased Cruz's protective instincts towards Megan; at one point, when Megan did not arrive home, he sent the entire police force looking for her when in reality, Megan's commute had made her seven minutes late.
Chief Cruz is stalwart and no-nonsense; as the newest head of police, he holds great aspirations for the police force to answer to every injustice San Fransokyo has to offer. However, he holds a negative opinion towards superheroes (whom he snidely labels as vigilantes) and believes they should not meddle with legal affairs if a police force is around to take care of them. While he does love his daughter, Cruz expresses his love in an extremely protective way. However in the episode "Legacies", he finally no longer hates superheroes as he accepts Big Hero 6 after helping them saving San Fransokyo from Trina.
Sometime after Cruz has moved back to San Fransokyo, he learns from his old friend Cass that she has a nephew Hiro Hamada, who is in the same age group as his daughter Megan. At Cass's suggestion, Megan is invited to Lucky Cat Café to meet Hiro for lunch.
One night, a fight between Big Hero 6 and High Voltage breaks out at a school dance hosted at Megan's high school. Shortly after the battle, Cruz arrives with the rest of the police and finds Megan with Hiro. He immediately goes to Megan to check if she is okay, and Megan introduces Hiro to her father, whom he recognizes as Cass's nephew. Megan tells her father about Big Hero 6, which Cruz scoffs at. When Hiro asks why Cruz hates superheroes, Cruz explains that they are vigilantes interfering with law enforcement, which he sees as a problem he intends to correct.
Cruz later endorses Megan's task in journalism in uncovering the true identities of Big Hero 6. He permits Hiro to visit Megan, guiding him to her room. According to Cruz, his team of best detectives had gotten nowhere on the team's whereabouts or the identities of its members but is confident that Megan will eventually have a breakthrough. He then goes to fetch some cookies for the two teenagers.
Later, when Hiro checks on Megan again, she reveals to him that she has marked a Big Hero 6 spotting on her map with dots and finds that the team appears most often at Krei Tech, SFIT, and Lucky Cat Café. At that moment, Cruz enters to commend "Super Sleuth Megan Cruz" for her work and snaps a photo of Megan's map to obtain his first step to unmasking Big Hero 6.
In "Legacies", he finally realizes the error of his ways and helps Big Hero 6 save San Fransokyo from Trina, and afterwards he learns of their true identities.
Nester Carbonell voiced Señor Senior, Junior in Kim Possible, another series created by Mark McCorkle and Bob Schooley.
His voice actor also voices King Raul in the Disney Channel show Elena of Avalor.