Señor "Sergio" Siniestro was one of the main villains in the TV Series El Tigre: The Adventures of Manny Rivera.

He is a very small 13-year-old Italian boy who was transferred to Miracle City from Italy. 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).