Sammy Jacobs is the main protagonist turned true main antagonist of the Goosebumps novel My Best Friend is Invisible and TV Series episode of the same name.
My Best Friend is Invisible (Book)
Sammy lives with his parents who are scientists. Sammy didn't really care for science, but he cared for science fiction. Sammy wanted to focus more on beating Roxanne (his girlfriend-ish friend) in a race. During that time he Suddenly met his invisible friend Brent Green and things suddenly turn out o.k. for him. But because his friend is "Invisible" and no one believes that Sammy has a new friend. He ends up becoming a laughing stock and also losing the race. Sammy tries to get rid of Brent just to make up for losing the race. Brent almost Killed Sammy in the process leaving Sammy to leave his room and turn the steam on. Sammy's parents think that he is insanely crazy and try to take him to a mental institute. Sammy wanted to know what Brent really looked like so he got his father's Flashlight and it was revealed that Brent is a human, Meaning that Sammy and his parents aren't. Sammy's parents explained that all of the humans are in the zoo because they are "Endangered Species" so Sammy decided to take him to where the humans are.
My Best Friend is Invisible (TV Episode)
In TV episode Sammy was played by Jon Davey. The TV Series Episode of My Best Friend is Invisible has the exact same plot as in the book it was based on. Except the TV series episode has a twist ending. In the book they said they were going to take Brent to the zoo, but in the episode Sammy and his parents surround Brent menacingly and then presumably kill him.