Complex sentence: Its main characters are George, Eric, Annie, Dr. G. Reeper (or Greeper), and Cosmos, the world's most powerful computer, which can make windows to look into outer space and doors which act like portals allowing travel into outer space.
Split into simpler sentences: Its main characters are George, Eric, Annie, Dr. G. Reeper (or Greeper), and Cosmos, the world's most powerful computer. Cosmos can draw windows allowing people to look into outer space, as well as doors which act as portals allowing travel into outer space.