Parodied: Eilrahc is only ever seen doing things that freak out Charlie and Charlie alone: his Trademark Favorite Food is something Charlie hates, his preferred mode of transportation is the kind of car you'd never see Charlie drive, he dresses more flamboyantly to contrast Charlie's more conservative attire or vice versa, et cetera.
Eilrahc isn't actually related to Charlie in any way.
Eilrahc stays in Charlie's head.
The director thinks this is the best way to make Charlie, who is his favorite character, more "present" in the plot at least for an arc or two without turning him into a Creator's Pet: have another, "evil" Charlie around as a major antagonist, which gives Charlie ample Character Focus and potential Character Development without it being unjustified or turning him into a Canon Sue.
The creator would like to have a fight between Charlie and Eilharc. Since Eilhahc is inside Charlie's head the creator make him his own body.
Lampshaded: "I didn't know any part of me was so evilk..."
Invoked: Emperor Evulz wants to use Charlie's combat ability against the heroes without having the potential troubles such as betrayal that an Evil Knockoff or such would bring under his command, so he engineers a set of circumstances that lead naturally to Charlie having one of these.
Defied: Charlie specifically avoids any situations that would lead to the creation of one of these.
Discussed: "Why do I have to do this myself? Because he's me. A very, very wrong part of me who has to be stopped."
Conversed: "Here we go, Charlie's going to have an evil version of himself running around for a few episodes..." "Until they kill the other one?" "No, until Charlie comes to accept that part of himself and absorbs it back into him, and we get some tired old message about accepting your whole self or some such."
Deconstructed: Eilrahc has all of Charlie's powers, memories, and abilities, but no moral code to hold him back from kicking dogs or even crossing the Moral Event Horizon. Either Charlie has no chance of stopping him because of the lines Eilrahc will cross to win, or on the off chance that Charlie does manage to win, his reputation is ruined and he probably also has to face the legal consequences for Eilrahc's actions.
Eilrahc has all of Charlie's powers, memories, and abilities... Up to the moment of their split. So if his memories of his other half's True Companions and other friends and family don't get him to do a Heel-Face Turn, then Charlie will rise to the occasion, take a level in badass, and ultimately defeat Eilrahc.
All of Charlie's powers except that Charlie can anticipate Eilrahc's every move having been him for so long, whereas Evil Cannot Comprehend Good.
Now that I can live without that insufferable main article, the whole world will soon be mine!