This character only appears in this timeline.
Arjah is the mysterious mastermind who seems to be behind a lot of the goings on in "The Alternative World". In the beginning, he appears as a shadowy entity who speaks through a piece of the Eye of God that Makoto Mizuhara had removed for study. Later on, he leaves the device and travels around as a shadowy sort of spirit creature. On the occasions when he appears to an individual within their mind, he resembles a gnomish old man, with pointed ears, tufts of white hair, and huge, piercing eyes. Just who or what is this creature?
That question is never really answered. Who Arjah is, and where he came from are two of the unanswered questions of "The Alternative World". One thing that IS made clear, however, is that he is a prisoner, forever trapped within the Eye of God that exists over Creteria. Forever, that is, unless he can get some poor, unsuspecting fool (or a series of such fools) to release him.
This desire, in fact, seems to be the driving force behind the entire series. Arjah was apparently the one to transport the cast to Creteria's world, all with the sole intention of maneuvering various individuals into ultimately unsealing and activating the Eye of God, an action which would finally allow Arjah to be free once again.
Initially, his machinations are relatively subtle. He manipulates Katsuhiko Jinnai with promises of untold power, something which always strikes a chord in the megalomaniacal Bugrom general. To do so, he claims the title of "Ruler of the Universe". Whether he actually holds, or even ever held such a position is questionable, but Jinnai eats his words up. (Editor's Note: Arjah does call him the Great Priest of Dimensions, but what that position does or did is yet another unsolved mystery.)
Once Jinnai (or, rather, Shayla-Shayla) has delivered him close enough, though, he emerges from the piece of the Eye of God and takes a more direct hand in manipulating events to his liking, first by forcing Makoto to extract the control codes for the Eye of God and then taking them from him. Afterwards, he outright possesses the mind of Gilda, and uses her body to set the final events into motion, activating the Eye of God and setting the stage for his eventual release from his prison. He's thwarted, of course, due to the efforts of our heroes, but, like any good villain, he sure caused a lot of carnage in the meantime.
The limits to Arjah's abilities are never explored in detail, but he seems to possess quite a few. While it is unlikely that he is actually the Ruler of the Universe, it can't be denied that he has a rather eerily accurate knowledge of the cast. He is able to recognize Shayla-Shayla, Makoto, and Jinnai on sight, and knows of the abilities each possesses that are useful to him. He also knows how best to manipulate them, using Jinnai's lust for power to ensure his cooperation, and nearly overwhelming Makoto's resolve with images of home… and of his lost love, Ifurita.
Arjah is also able to project his image and voice from Creteria's world to El-Hazard through the dimensional component that Makoto removed from the Eye of God. Since Arjah is trapped within the Eye of God on Creteria's world, this provides yet another hint that the two Eyes might be connected somehow.
Though sealed within the Eye of God, Arjah does have some freedom of motion, as a spirit. He is able to travel freely around Creteria's castle as a sort of shadow. In that form, he is able to enter people's minds at will and, in at least one case, is able to overwhelm that mind entirely, moving the owner's body as if it were his own. Nevertheless, this freedom is not enough for him. Nothing short of his complete escape from the Eye of God will satisfy him, and if the annihilation of Creteria and all of its people are what it takes to accomplish this... well, that doesn't bother him a bit.
By the end of the series, Arjah has failed, thanks to the efforts of Makoto and the three Great Priestesses. Just when he'd managed to open the way to freedom too, poor guy. That's gotta suck. The implication given is that Arjah is now completely sealed away, with no hope of ever escaping again. Why he was only partly sealed before is unknown. Perhaps the condition his prison was in (i.e., in many small pieces) is the reason.
Arjah is probably the greatest mystery presented by "The Alternative World". That he is important to the plot of the story and to its resolution is undeniable, but with his origins never made known, he seems almost more of a plot device than a character, thrown into the mix so that the heroes would eventually have something heroic to do.