In the first two films, he was a minor character. In the first film, he was seen interacting with Chloe. He and his friends were at a party wondering where Chloe was and that the party was going on a magizine. In the second film, he was seen eating cake at Papi and Chloe's doggy wedding.
In the third film, he is more of a supporting character. Throughout some of the film, he helped Papi plan Rosa's fiesta for her sweet 15 (in dog years). Sebastian auditioned different bands and adored some of them, though Papi was displeased with all of them, until they both found the right one. Later, Papi informed him that he was suspicious of Oscar's hidden agenda involving Jenny. In the end, he was seen at the party with everyone else.
Committed mass genocide against multiple minorities, as stated in "Beverly Hills Chihuahua 3: Viva la Fiesta."