Lore:

Slight spoilers for the entirety of Nodkrai and Varka's story quest ahead!

Gilles first met Seamus after Varka returned to Mondstadt from Nodkrai accompanied by that beautiful charming man. Gilles, already a member of the church as an organist, was the one to introduce Seamus to the church's teachings. He soon developed strong feelings for Seamus but never confessed, as he knew that Seamus was interested in Frederica, who he eventually began dating.

The unsaid confession continued to fester within Gilles, combining with feelings of inadequacy as a follower of the God of Freedom for his inability to let Seamus be "free", and turned into something desperate and bitter that he hid from everyone. When Seamus asked Gilles to officiate his marriage to Frederica, he agreed, wanting nothing more than for Seamus to be happy, even if he deep down objected to their marriage..

Shortly after the newlyweds said their vows, Gilles ran away from the wedding and boarded the first ship heading far away from Mondstadt, leaving without a single goodbye. He found himself in Nodkrai where the turmoil in his heart attracted the attention of the Wild Hunt. Mistaking Gilles's heart for its own, Rerir's fractured conciousness consumed it.

The news of Gilles's disappearance dealt a heavy blow to Seamus and soured the memories of his happiest day, which may have been one of the contributing factors to his eventual divorce with Frederica. With each failed attempt to locate Gilles Seamus began losing hope that he'd ever see his best friend ever again, but the fact his body had never been found kept it from dying altogether, despite everyone's advice to let him go.

Many many years later, after hearing of Varka's upcoming expediton to Nodkrai, the hope in Seamus's heart was rekindled and he decided to look for him one last time before finally coming to terms with the worst.

Rerir's eventual revival awoke hoardes of Wilderness ghouls among which, to everyone's horror, was the shambling corpse of Gilles. Despite the knights' warnings, Seamus refused to let this remnant of Gilles be killed. He caged it in a wooden crate and attemped to draw Gilles's consciousness out of the depths of this monster by recounting the stories of their youth and friendship. During this time, Seamus came to realize that his feelings for Gilles run deeper than he ever admitted to himself. Unfortunately, the ghoul remained unmoved.

Unsatisfied with this, desperate, and growing distrustful of the rest of the expedition, who considered Gilles's presence a risk and insisted he must be killed, Seamus separated from the group and travelled South, meeting up with Albedo and the others to work on a solution to this situation. However that had to be delayed because of the growing danger from Dottore. Under the guise of helping develop the World Formula, Seamus used the Heretic's knowledge to develop his own plan. After returning to Mondstadt with Lohen, he his Gilles beneath the cathedral, waiting for the right moment to act.

The time came when many of Varka's friends gathered at Windrise to bring the grandmaster forth from the Ley lines, leaving Mondstadt unattended. The group returned to find the sky above the city has turned an ominous crimson and the melody of a wedding march blared from the church's organs. Seamus laid bloodied at the altar and above him, sat playing the organs, was the horned figure of Gilles.

Through a process of alchemical marriage, Seamus extracted his own heart and gave it to Gilles to return his memories, who in turn swore to keep it beating for Seamus. Bound together forevermore with not even death able to do them part, their inability to let each other go has led them to defy the laws of life and death.

Marriage:

The main theme of Gilles's and Seamus's designs and story is the concept of marriage. Now, you must forgive me, because I actually know very little about the subject, especially the concept of alchemical marriage or alchemy as a whole. In short, drawing from my limited understanding, Gilles represents the white queen, Seamus the red king.

The queen, associated with the Moon and silver mercury, represents the feminine and the soul. Her dominion is emotion and wisdom. Indeed it was Gilles's emotions that started this all, and he is the more sensitive of the two.

His design is asymmetrical, made up of softer curved shapes. He wears his heart front and center, accompanied by smaller heart motifs, notably silver heart embroidery on his sleeves (reference to wearing one's heart on their sleeves). A torn bloodied skirt peeks out from underneath his robes meant to allude to a wedding dress but also to an alb. The design of his cape resembles the wings of a luna moth, further reinforcing his connection to the moon.

Seamus on the other hand represents the red king and spirit. His domain is the masculine, red sulphur, and the Sun. Of the two he is more often the voice of logic, able to see through Gilles's melodrama.

His design is symetrical and comprised of sharp pointed lines. The only heart present on his design is the one in the middle of his chest which is empty, signifying his physical lack of one. The white harness binding his body, resembling a ribcage, reinforces the idea of an empty chest cavity as there is nothing to be found behind it. A dark waistcoat and a shape resembling a bowtie at his neck allude to a groom's attire. Gold rays decorate his mitre, forming a crown of sorts. The astrological symbol representing the sun (☉) decorates his design, notably on his shoulders and on his back.

In many depictions a dove blesses the union between the king and the queen. Dove wings are present in both designs and Gilles's colour pallete is actually based on the bleeding heart dove.

Sin and Heretics:

A second large source of inspiration for their designs was actually Dottore, believe it or not. After all, it was his knowledge that allowed for them to be truly (re)united. Because they're minor details, I went ahead and prepared a simple visual guide.

But why Dottore, I hear you cry. After all, it was Sandrone, Albedo, and the rest, who compiled the knowledge required for the World formula.

Well, firstly, I believe in giving credit where credit is due. Secondly, Seamus and Gilles have both Sinned.

The Devil and the Church of Favonius:

While there are mentions of Hell in the game, we don't really know much about the workings of the church, so I will be getting into some strong headcannon territory here.

From what we know, the verion of Barbatos that the church worships is much different from how he actually acts and feels. It's safe to assume that the church's God is much stricter, even to the point of its members or other believers developing pervasive feelings of guilt, as is the case with Gilles.

Following his ressurection, as all of his feelings were returned thanks to Seamus's sacrifice, Gilles experienced such a strong wave of grief and loathing that it physically mutilated his body to resemble that of the Beast. He cannot stand thinking what he put Seamus through, he cannot stand thinking what Rerir made of him, and he cannot stand thinking what he made of Seamus. They're both undead now and forcibly preventing the freeing of a soul into the afterlife is a sin. Gilles could choose to stop their shared heart from beating but how could he take someone as wonderful as Seamus away from those who love him? (And how could he ever allow himself to let go of the man he loves? Gilles is far from selfless, though he feels guilty for that as well.) Surely a creature who commits sin regardless of whichever action they choose must be of the devil.

In tarot, the devil card represents dependency, lust, obsession, hedonism, addiction, and inability to let go. Depicted underneath the Devil are two figures chained at his feet. They are trapped in servitude, or perhaps slavery, to their desires.

Similarly, both Gilles and Seamus are incapable of letting go of one another. Gilles, deep down, could not bring himself to accept Seamus's disinterest, while Seamus couldn't bring himself to accept Gilles's passing. Representing the desires that bind them are the chains that fasten their waistcoats to their bodies.

So, much the same way that Dottore attempted to usurp the world for his own desires, Seamus and Gilles defied not only their church but the workings of the world itself to have one another.

(Now, I do not know if the church of Favonius canonically believes in any figure equivalent to Satan. BUT in this AU they do, and it's just an unfortunate historical misinterpration of Morax. Think about it: powerful, sometimes takes the form of a serpent or other beast, tied to wealth, binds people in contracts... Maybe it started as a tale told by Barbatos a long time ago about one of his Archon friends but over time, as it was retold again and again, some of the details were lost or twisted. The image of a mountain goat became associated with this figure as a creature that climbs towards the heavens instead of being satisfied with what is found on the earth, the same way people strive for what they cannot have. But that's not a detail Gilles is aware of.)

Trivia and anecdotes:

Here is a list of less important information to simply flesh Gilles out. Eventually I would also like to create a set of false voicelines for him, as if he were a playable character. They will be unvoiced, but I imagine him as a very soft spoken person with a quiet, slightly breathless, and nasally voice.

• As I've alluded to already, Gilles is a very dogmatic character. Though reviving the dead is nothing new in Mondstadt and actually seems to be a growing trend (Durin, Varka...), Gilles still feels like his second chance at life is, at the very least, undeserved. Noticing this inner turmoil within him, Venti revealed his true self to Gilles in hopes of easing his guilt. Sadly, not even an offical pardon from his very God could truly erase a lifetime of conditioning from the church. If anything, the knowledge that his God is always nearby adds to Gilles's anxiety.
• He is an excellent organist. His best performances include: improvised sonata #6 performed at 3:14 AM, improvised sonata #8 performed at 2:56 AM, and improvised sonata #11 performed at 4:13 AM. His latest work, improvised sonata #13, promised the great return of this nighttime musical genius, but was (un)fortunately interrupted before the fourth movement by Sister Rosaria. Since then, Gilles was banned from playing during the witching hours of the night for good.
• Offically speaking, Gilles is not an actual cardinal, despite his nickname as the "Cardinal of Nighfall". The term seems to have been coined by Venti, who wasted little time before turning their tragic tale into a touching piece of musical work. Gilles attempted many times to correct people on this mistake, to no avail. Seamus offered to promote Gilles to the rank but Gilles denied on account of lacking necessary experience.
• Gilles's birthday is on the second of November, on which people remember those who have passed (All Soul's day). His ressurection was on Hallow's eve, when the veils between the living and the dead are said to be the thinnest. Seamus insists on celebrating both.
• The relationship between Gilles and Seamus's daughters is rather strained. He and Jean don't get to talk often and despite Barbara's attempts to connect, Gilles remains rather avoidant, fearing she hates him for causing their father so much grief.
• Of all the nembers of the church aside from Seamus, Gilles gets along with the best with Dahlia. Dahlia's playfulness draws him out of his shell and allows for some of Giless's own theatrics to show through, while Dahlia on the other hand enjoys hearing about Gilles's life and troubles, and even more than that enjoys trying to solve them, secretly passing on Venti's guidance to Gilles as well.
• Varka likes to joke that the cardinals stole his spotlight becuase he too was revived on the same day as Gilles, yet the people of Mondstadt seem to forget that. Gilles wrote him a lenghty apology letter. In response, Varka simply clapped him on the shoulder and said he'd rather Gilles invite him for a beer.
• Gilles, as his name might suggest, hails from Fontaine. He ran away from home at a young age from a neglectful family, searching for order and stability. The House of Hearth offered him a place within its walls but before he agreed and signed his life away, the wind blew a torn page from a book on the proper conduct of Barbatos's clergy into his hand. Gilles was immediately drawn to the detailed instructions and boarded a ship for Mondstadt.
  To his dismay, he found out that the book the page was from was outdated by then and the rules for the clergy were much looser these days. Still, the church recognized his eagerness to serve and took him in.