Invader Skoodge and Zim are both Irken Invaders, and both are hated by the Almighty Tallest, for various reasons (primarily, their short statures). They have seemingly formed a friendship, despite the way Zim treats Skoodge.
Skoodge seems to be unaware that Zim's mission is a lie, or at the very least Skoodge doesn't want to tell Zim, as Zim tends to get violent whenever he is upset. Whatever the case, Skoodge is loyal to Zim, despite the fact that the latter was more than willing to sacrifice Skoodge to a Hogulus in order to get what he wants. Something similar also occurs during a flashback in the unproduced episode "The Trial", when a smeet Skoodge joins a smeet Zim in playing hooky and escaping with him to see the then off-limits surface of the planet. During the escape, however, Zim used Skooge as a distraction for a security droid, and paid no mind when he screamed for help.
Skoodge, like Zim, seems to be unaware that the Almighty Tallest hate him, even though they shot him through a cannon into the planet Blorch, home of the Slaughtering Rat People. However, Skoodge somehow survived this, and joined Zim on Hobo 13 to train with him and also undergo a near-death experience, though he survived this one as well. When Skoodge found out that the Almighty Tallest actually hated him, he travelled to the planet Earth and took up residence in Zim's basement. He was to have had a much larger role had the series continued, and was to have helped Zim with his schemes.
A sketchy plot idea for the series finale, "Invader Dib", would have had Skoodge help Zim rescue Tenn from the Meekrob (and inform Zim about the battle on that planet), but it is unclear what would have become of him afterward.
Zim sees Skoodge as a pitiful excuse for an Invader, and calls him gullible, and he seems to be aware that the Almighty Tallest hate him. He often abuses Skoodge, and if the series were to continue on, he would not be very welcoming when he finds him in his basement. He thinks that Skoodge was just lucky when he conquered his assigned planet Blorch, despite Skoodge being an excellent Invader.
Zim is occasionally described as Skoodge's best friend, although he doesn't act like it, as he is quite frequently cruel to Skoodge. He feels that Skoodge is rubbing his victory on Blorch in Zim's face, and his resent for Skoodge increases every time he encounters him. However, by the time "Invader Dib" happened (in a very sketchy plot for it, at best), the two would have worked together to save one of their own, and Zim would later claim the throne of Irk as the next Tallest.