Character Information:
Michel: Name Notes
This one is complicated. First, a word about katakana. Japanese writing uses katakana, a syllabary of simple shapes that represent simple sounds (sounds, but NOT meanings) for a variety of purposes, including the approximation of foreign words. 
is written in katakana, and is the Japanicization of a *foreign name* based on the Hebrew name "Michael." The problem is, I have seem used to represent the names Michel (French), Mikel (Basque), Mikkel (Scandinavian), Miquel (Catalan), and Miguel (Spanish and Portuguese). All of these names are variations of the name "Michael." So when I first starting researching the name , I had no idea which of the above names Yokote Michiko had intended - it could have been Michel, Mikel, Mikkel, Miquel, OR Miguel!
However, the release of the second Pure DVD box included a character design booklet in which Rihito's sister's name was written out as Michal. Michal is also a feminine variation of "Michael." Since our psychotic angel's name is supposed to be only one letter different from Michal's name, that probably means that our angel is supposed to be named Michel. Michel is a name that, like Michal, is a variation of "Michael." It is also a name that the katakana characters are often used to represent. Finally, like I mentioned before, Michel has only one letter that is different from Michal, a pattern that is also true when these two names are written in Japanese.
Names Notes, Part Deux: There are some people that say that IS supposed to be Michael. Unfortunately, that's incorrect. When converted to katakana, the Hebrew name Michael becomes 
. Notice that this name is written with different characters. However, there is no question that represents a name that is BASED ON "Michael," whether it is used to write Michel, Mikel, Miguel, or any of the possible names that I listed in the above paragraphs. But it is not actually "Michael."
Name Notes, Part Trois: "Michael" (and also "Michel") means "Who is like God?" In the Bible, Michael was an archangel, and leader of Heaven's celestial armies. Michael was the angel who defeated Lucifer during the Great War, and who cast him down into the Abyss.
Character Information:
Michel: Spoilers
So, what is Michel? Highlight to read the biggest spoilers below:
Michel is actually a scientific creation - the result of DNA from the fossilized remains of one of the "ancient ones," injected into the body of a human girl with Panthalassa blood, one Amagi Michal. However, through the body of this fake, mutant Michel, Fuku-chan had hoped to revive the spirit of the true Michel, the king of the ancient ones. (It is the true Michel whose physical remains are preserved in a fossil stored in the bottom of fake-Michel's castle.) It is Fuku-chan who reveals the story of the ancient ones to the mermaid princesses: "This planet changed itself into an environment where humans could be born. This planet betrayed us for the sake of those lowly humans! Our king Michel prayed so much for the sake of our race - so much so that his power drained away." Fuku-chan and the fossilized remains of Michel were left behind by the ancient ones, so that one day they would awaken in order to restore the ancient kingdom, and the ancient races, back to life.
However, the plan doesn't work - the fake-Michel goes crazy when he discovers that he isn't the real Michel, and all of Fuku-chan's plans go haywire. Finally, however, the spirits of the ancient ones appear before Michel and apologize, saying that they were afraid of dying out, and thus placed too large of a burden upon their king and his servant (Fuku-chan). The ancient ones explain, "But yet today, we still exist. We still live on, as a warm love inside of every living thing. Like this, we still live on. So you may stop now, Michel. Become a spirit and return to us." Michel protests that he is only a "fake," but the ancient ones answer, "No. You, child, are one of our family." Having finally found those who call him "family," Michel is overjoyed, and he and Fuku-chan join the ancient ones as they fade into the sky. As they ancient ones leave, they say to the mermaid princesses, "We are the ones who made Michel suffer. The sins that this one and Fuku have committed are our sins. Let us wait, together, for the right time to face our judgement. If we are ever truly reborn, we will carry with us the heavy burden of our sins, but at the same time we will search for the new meaning of our existence. Let us wait for that time, with faith in the future in our hearts." Thus ends the story of Michel.