He is an autistic kid who has a passion for trains, being also a very good listener and observator, noticing everything that is around him, as seen when he talked to Lionel when they were going to Nancy's playhouse in his debut episode "Nancy's New Friend."
"He is very cool because he sees the world in a whole different way," that is basically what Lionel thinks and that is why Lionel consider him his favorite cousin.
And about his likes and dislikes, he loves smooth peanut butter and doesn't like crunchy. Also, he can be very sensitive due his autism and when it is too much, he runs away, but after a little quiet time and a talk to his mom, he comes back and ready to play!
Sean is a fair-skinned young boy who has dark brown hair, green eyes with thin eyebrows.
He wears a blue jacket, grey joggers and red running shoes.
He is voiced by George Yionoulis who also, according to the Hollywood Deadline article about the season two episode "Nancy's New Friend", inspired Matt Hoverman to make said episode because of his viral video "My Autism - By George.", where he explains to his fourth-grade class how it is like to be autistic.
According to Matt Hoverman, Sean is the first autistic animated character of Disney.