Sergio is a very small 13-year-old boy who recently moved to Miracle City from Italy. On his first day at Leone Middle School, he foolishly wore his beloved cowboy costume, complete with broomstick-horsey. Humiliation, unintentionally led by Manny, ensued and Sergio swore revenge on Miracle City, especially Manny. Sergio's supervillain suit of choice is a 10-foot tall robot suit. When inside the suit, he's unrecognizable and poses as an adult cowboy supervillain Señor Siniestro. Sergio also builds other cowboy robots to assist him in his villainous adventures. Señor Siniestro is usually found spewing cowboy lingo in an absurdly inaccurate Texas accent while looking for his foe, Manny Rivera.