He is a very small 13-year-old Italian boy who was transferred to Miracle City from Italy.
He was voiced by Jeff Bennett. who also voiced Coverton from Monsters Vs Aliens Tv Series, Tong Fo from Kung Fu Panda: Legends of Awesomeness, Dorkus Aurelius from Planet Sheen, Boog, & Traloc from Tak and The Power of Juju Tv Series.
On his first day at Leone Middle School, he foolishly wore his beloved cowboy costume, complete with broomstick horse, Mr. Broomy Horse. Humiliation, unintentionally led by Manny, ensued and Sergio swore vengeance on Miracle City, especially Manny. Sergio uses a 10-foot-tall (3.0 m) robot suit to pose as an adult cowboy super villain named Señor Siniestro. Señor Siniestro is usually found spewing cowboy lingo with an absurdly inaccurate Texan accent while looking for his foe, Manny Rivera. He was also one of the competitors of the Super-Villain Grand Prix and kicked the other competitors out of the race and got crushed by a crocodile and finished 2nd place over a crocodile. He also tried to make Manny and Frida late for school so they would be expelled. He also forced the students to build him a robotic horse suit which was destroyed by El Tigre. He has a huge crush on Frida. His supervillain name means Mr. Sinister. He is friends with Dr. Chipotle Jr. and knows his real name. They sometimes play video games in his lair (which in reality is their actual creations fighting behind a screen to make it look pixelated).
In "El Tigre, El Jefe", Sergio planned to get revenge on Manny (who is El Tigre) for humilating him on his first day. In "The Late Manny Rivera", I
n "Fool Speed Ahead",
In "The Good, The Bad, and The El Tgre", He appears as one of the villains who tried to win Sartana's special tournament. However, Later on, its revealed that the other villains are only chained which because they are Sartanna's Enemies for an Unknown reason, & its revealed that it all part of Django's plan, it is unknown why Sergio is an enemy of Sartana.
In "The Return of Plata Peligrosa",
In "Love and War",
In "Wrong and Dance,
In "No Boots, No Belt, No Brero",