Luckily I just so happened to be visiting home, and I had a stack of old comics readily available to scan (read: photograph and then touch up in PS). So I went ahead and uploaded some old drawings to satisfy your curiosity.
Warning: really terrible elementary school art ahead!
This was the first recorded appearance of the villain, I believe. This was in fact drawn NOT by me but by my good childhood friend Yoshi. We both drew comics and made a more-than-regular habit of borrowing each other's characters and expanding each other's canon. Ultima started off as such a snag by myself from Yoshi.
This comic was one of many many series which he started, though never completed. Usually his protagonist was Yoshi, the Zelda/DBZ-like legendary hero. This was to be the story of his 5th descendant, Moshi. I went on to make use of the name Moshi in Phanta as the samurai ancestor of the protagonist, as a nod to this. It's especially apt because "Moshi-Moshi" is a phrase used in Japan to answer the phone, based on an urban legend that if one were a ghost they would only be able to say "Moshi" once. So, saying Moshi twice proves you're not a ghost to the otherwise unaware caller.
This was the first page, also drawn by Yoshi.
Rule 1: Eat healthy foods (unless you are Will) [[disliked classmate]]
Once the world was made...
By Ultima, that holds the Ultima power
40000000 years later...
(Moshi: This is it...)
One day, Ultima turned evil (Ultima: Mwahahahaha)
Then the world turned evil... (Man: Eek!) [Comment: Wash your hands after you kill someone, or if you are killed]
Et cetera. I took him and managed to make him even more generic for my purposes. It took a while to find a drawing that actually featured a full shot of the guy: here. This is just a piece of the 100-ish page spanning final battle between him and "Greg," the OTHER Zelda/DBZ-like legendary hero. I promise you that this page probably had the most talking in it of the entire fight, because the vast majority went something like this.
And then later on I sort of "updated" the design a bit to make him ever so slightly cooler-looking.
His story as I told it was generally that he was this super-great god like demon who created Earth, but then a mysterious black liquid from space turned him evil. You know, like Venom in Spiderman. So he goes nuts. Anyway, it turns out he doesn't have the full powers of a God anymore and that the power was kept in this big castle locked behind a sealed door than could only be opened by killing the 5 or 6 "chosen ones" who kinda represented the different seals on the door and passed on this seal to their descendants. So naturally he goes and kills them one by one, including the main character, Greg.
Greg has to go through a whole bunch of trials to get back from the afterlife, and arrives just in the nick of time as Ultima is about to get his grubby demonic hands on the god power. Just then, the famous/best wizard Sauron shows up in the nick of time as well and splits up the energy, casting it in pieces to the ends of the Earth. A piece of it goes to Greg, which makes him way stronger and a match for Ultima. Ultima takes out the super wizard Sauron in frustration, but then is defeated by Greg in a big final battle.
And then when he inevitably came back in later comics, the heroes of the day had to go on a big Zelda-style adventure and collect the pieces of the god power stuff to fight him.
That pretty much covers it! All this stuff about Ultima isn't without parallels to the big bad in Phanta, though this is probably mostly because as a whole Ultima is pretty generic. When the game is finally finished and everyone gets to see how the story unfolds, you might enjoy finding the characteristics and plot stuff that crosses over between the two.