Alex Rider is a title character and the protagonist of the popular "Alex Rider" novel series by British author Anthony Horowitz. He has also been featured in three short stories written by Horowitz based in the same canon as the series; "", "" and "".
Alex Rider is the main character in three short stories written by  Anthony Horowitz.
The context doesn't state whether Alex Rider is a main character in the three short stories, only that he is the main character in his Alex Rider novel series.