Excellion

Excellion (sometimes referred to as Excellion Caelis and rarely as Excellion Lucis Caelum) is a son of genesis regalia user that has been present in history for an indefinite period of time, existing before the creation of reality according to The Composer. Not much else is known about him or his origins.

He's the literal combination of two previous regalia users (first and fourth), and as such he's both female and male simultaneously but not necessarily physically. It is worth noting no one remembered ever seeing his female form, in fact, all the information in this paragraph was unknown even to his family up until the Rising of the God-Shattering Star.

Appearance
Excellion is usually depicted as a young man or woman with extremely pale skin and pure white hair, usually wearing clothing with black, white or bright colours. His eye colour is usually either golden or red. When on his male form has the Mark of Regal Radiance on his right eye, a symbol of true divine power. If she's on her female form, the Mark of Regal Radiance is inverted. At any time they can choose to show both marks at the same time, showing a symbol like that of the Book of Genesis.

His appearance is not constant due to shapeshifting, which is also what he uses to alter his eye colour.

His wearables and eye colour(s) may change when driving.

Tatels once turned back time on one of his arms, that arm turned completely black with a white aura until it was manually shapeshifted back. This is probably an interaction with Excellion's timeless nature.

When utilizing Twilight Trigger, he sprouts tree-like wings made of a substance that looks like ink. These wings can be of variable size.

Personality
Excellion is a terrifying entity capable of adapting their personality to fit every single situation depending on his objective relative to the person they are talking to. We do not currently know if Excellion has any sort of "true" personality or if his personality is just naturally adaptable as previously described.

It'd probably be more useful to check the relationships tab to see how this unorthodox personality has affected the world.

Background ~ The Progenitor
Not much is known, he is a dimensional traveler that garnished his spells and abilities through exploration and meeting people. However, now we know that he is the one that caused most (if not all) of the incidents shown thus far and he is also the one that wrote the Book of Genesis/Akashic Records. It is unknown whether Excellion also planned for Lucisfear's creation.

He is originally a fusion of the First and Fourth Regalia Wielders performed by Sentient Genesis and a being that had their existence predefined before the creation of the world.

Excellion has admitted that he committed all crimes penalized by law and wiped out entire civilizations for reasons unknown. On the flip side, he has also contributed to an incredible amount of all developments and events ever made (and he has at least foreseen all events as evidenced by the Book of Genesis), thus, utilizing divine techniques like Shiro's Regalia abilities, he can be traced backwards as the origin of many events and discoveries even including the times of the original First and Fourth Regalias Wielders. This has earned him the titles "The Source of every breakthrough" and "The Progenitor".

Special mention to the following scene(s)
Monsoon (also called Tael Lichtflügel, aka CT before CT was a thing) once hit Excellion with a blade that was prepared to apply the concept of death on its victims. Excellion's body rejected the concept and the Jailor was promptly deleted from existence (not killed mind you, deleted) in a surrealistic sequence that engulfed that dimension in the "ink" shown during Excellion's Twilight Trigger.

Quotes

 * "I know everything, there's nothing i don't know"
 * "Everything can be predicted if you have a starting point to gather information from, even parallel timelines"
 * "I wrote the ending a long, long time ago" (talking about their writing of the Book of Genesis and Lucisfear)
 * "I'll be your bitch if you want me to~" (mocking TAEL's manner of speech)
 * "I am unimpugnable you fool" (mocknig TAEL again)
 * "Oh please, I'm just an inoffensive cute albino girl" (mocking her own existence)
 * "Careful, the shippers will get excited" (meta comment mocking certain real fandoms)
 * "My answer has, but you wouldn't remember" (to CT referring to the memory loss surrounding them)
 * "I live only for the world that beholds me" (to CT about their purpose)
 * "Thank you for taking part in our world Tael" (thanking CT)
 * "I decided to create something that was not from this world, something truly eternal. That which my father couldn't make" (talking about Kuon)
 * "But are you really her mother?" (making the Manager question her identity concerning Ciel)
 * "Are you always this dishonest?" (questioning Ishtar's demeanor)

Trivia

 * Excellion's "original eye colour" is red.
 * Excellion is much more powerful than other Regalia wielders due to his synergy with the Rapture, his having two regalias, and Twilight Trigger.
 * Twilight Trigger is a provisional name because I couldn't come up with a name that had the meaning I wanted.
 * Twilight Trigger (the name) is a reference to Gunvolt Chronicles Luminous Avenger's "Darkness Trigger".
 * Mind melder now works like an actual mind melder, because in Nova's case (in Azure Striker: Gunvolt 1) it was localized from "Psycho Fusion".
 * Also, psycho fusion now works how mind melder seems to work.