Action Girl: Most of the named women characters are one, except the ones who were first seen residing in Avalon. Rhiannon gradually turns into one near the end of the anime series.
A-Cup Angst: Epona is especially conscious about her size to the point of being so embarrassed as to having to resort to a Love Potion just to bed Arawn. Yes, she even rambles about it during her H-scene. See Red String of Fate below.
Berserk Button: The otherwise snarky Arawn hates it when people are willing to die or enslave themselves for immaterial concepts like chivalry and honor. This is likely due to how his creator/father Angel sacrificed himself to give humans a chance for freedom that, thanks to the Angels, the previous races of the world never had.
Arthur's gets pressed in episode 15 when he first sees Arawn as a white spirit, remembering it was one who killed his father in front of him when he was younger.
Big Brother Instinct: Arthur doesn't trust Arawn at first. Hard to blame him, since his first sight of Arawn is as a huge demon, right before he morphs into a human-sized figure, and thought that his sister Rhiannon was in trouble. And although Arawn actually saved her, he wasn't entirely sure of it at the time.
Call Back: Arawn is known as the "Great Demon King", presumably because he's a Fallen Angel. Then a flashback reveals that while Arawn was "dying" the first time, Pwyll utters the words: "...My friend. My Great King.".
Complete Immortality: Angels don't appear to be capable of dying either due to age or violence. Myrrdin is the sole exception, which was through sacrificing his very being. Arawn was 'killed' in combat, but it just meant he had to spend a thousand years sleeping. Even as a fallen angel he claims he can't really 'die.'
Compressed Adaptation: The manga, being only 15 chapters long, axes entire parts of the story and quickly brushes over others, like Arawn and Myrddin's relationship or Arawn's past as an Angel.
Dark Is Evil: You might think that with Light Is Not Good in play Dark Is Not Evil would accompany it. It doesn't: Unholy powers really are bad and while Arawn looks dark and forboding, he never displays any dark powers. As a matter of fact, the one sending out unkillable undead are the angels themselves.
And Gaius's lieutenant, Decimus; though in the anime he's clearly ready to kill them (or die trying, anyway) after Gaius is mortally wounded, Decimus obeys his commander's final order: defect to Arawn's side and help him create a better world.
Contrast with Light Is Not Good — the Holy Empire, though it's obviously corrupt from the start and thus not ever really light; they've got nothing on the Angels themselves, of course.
Dead All Along: The emperor is long dead. Lector just talks to him for shits and giggles and interprets the skeleton's silence however he pleases.
Deadpan Snarker: Arawn will sometimes lapse into these during less serious moments.
Depleted Phlebotinum Shells: The holy element Electrum burns Arawn on contact (getting shot full of it was what killed him the first time). In-game, any equipment made of Electrum doesn't enhance the stats of any elves, dragons, or demons it is given to. In human hands, however, they're the most powerful things available at the store.
Depower: What happens to Arawn after High Priest Drwc resurrects him too early. As a result, the legendary Demon King is only slightly stronger than a normal human and actually hurts himself a little when he punches a skeleton so hard it shatters.
In the game he instead rips Drwc apart with a sweep of his hand as if his fingers were solid claws. Again, he has some minor complains about it afterwards.
The Fair Folk: Subverted. When the boat first lands on Albion, Ogam mentions that they best be careful because of the "Good Folk". They actually turn out to be very friendly. It also helps that they adore Arawn.
Fallen Angel: Arawn cast himself out of heaven in defiance of the other eleven angels who desperately wanted him to stay simply so there would be twelve. With his new freedom, he brought freedom of their own to humanity. His first and only act as a White Angel was to give a wreath to a young girl, which stripped his divine powers. He can forcibly regain them, but it hurts even more than it did to cast them away before.
Final Speech: Combines with Almost Dead Guy during Gaius' last moments in the anime for an impressive amount of time; it would almost have been ludicrous had the character not been so badass that one could buy Gaius' willpower holding out long enough to make that speech.
Go Through Me: Rhiannon does this a few times to protect Arawn from Arthur.
Gratuitous English: "Apple Pie". When your city's name is a throwback to the fact that its orchard is full of apple trees, it only makes sense that this gets made... often.
Have You Seen My God?: Watos, the actual creator deity, vanished almost as soon as the universe was created and hasn't been seen since. The angels try to follow his will, but they haven't the faintest idea of what it actually is. They suck at knowing the right thing to do and as a result are vicious and keep destroying the dominant civilizations.
Heel-Face Turn: Taliesin is briefly a minor antagonist, but since the entire point is to recruit him and his tribe it's obvious that he's going to join them eventually.
Honor Before Reason: Many of the Gael Tribe, but Arthur takes it up a notch. When told to duel a wounded foe, he dislocates his own arm to even the field. Arawn frequently remarks that Arthur's stubborn insistence on rules and oaths can do more harm than good, especially when dealing with human lives.
In the game, there is a law that punishes adultery with a dislocated arm, so Arthur runs away with this and casually asks Morgan (who has declared herself married to Arawn) if her breasts get in her way when fighting before grabbing them just in time for Rhiannon to see the act.
Hot-Blooded: Arthur. A significant part of the plot revolves around the whole cast just trying to stop him from going overboard. Following Character Development though, he's definitely more matured and composed by the end of the series.
Morgan falls into this trope from time to time as well, and nearly gets them caught when she can't shut up while they're exploring Londinium for the first time.
The same can be said of Rhiannon, who looks like an adult Primula, Pwyll's human wife.
Not surprising, considering that in the very beginning of the game, she tells Arawn that her true name is Primula.
Idiot Hero: Arthur to some extent. Comes to a head when he throws Arawn's caution to the wind and leads reckless charge against the Rubrum, nearly losing all him men. This is frequently lampshaded in the game.
I Know Your True Name: Knowing the name of a spellcaster gives you complete control of them. Riannon is forced to give up her name to a priest early on and Arawn has to break the control before killing the priest. Riannon in turn figures out Arawn's own name and almost blurts it out, much to his irritation. However, apparently it wouldn't matter even if she did know since he's above the ability of a human to control.
I believe, that in the VN, while saving Rhiannon, Arawn mentions that the whole "true name" thing works only as long as you believe in that you should obey, and he's disappointed in her being stupid enough for this. And then he snaps her out. By hypnosis. In a matter of seconds. And then rips apart the one who threatened her (in the True Name Slave state she would die along with him) with his bare hand.
Arawn states very early in the game that "true name" control doesn't actually work on humans. It's implied this is due to his own sacrifice which gave them free will and broke heaven's control of them. It is very clear that the "true name" control works on everyone else as Arawn himself was taken control by Myrrdin when the latter uses his true name Lucifer.
Keystone Army: Upon defeating Gaius, the remnants of the Empire's forces sieging Avalon surrender immediately, at his dying request.
Large Ham: Morgan sometimes does this. One prominent example is in episode 16, after Ogam tells them not to move their forces towards Avalon for four days.
Leeroy Jenkins: Arthur is worse off than Oboro in this, but at least he gets a clue quickly enough.
Morgan can be prone to these as well.
Lethal Chef: Subverted in the game. While Octavia and Morgan's cooking looks something like slop (they just ground up all the ingredients and threw them into a boiling pot), it actually tastes pretty good.
Light Is Not Good: Holy power is associated with purity, precision, belief and perfection, but when taken to extremes it also promotes blind faith, arrogance and self righteous zealotry. The Empire is strongly associated with holy powers, which makes its more moderate members like Gaius and Octavia uneasy. They're being led by Lector, one of the angels, all of whom have fallen into zealotry.
Loophole Abuse: In episode 16, Ogam tells Morgan not to move their forces for four days. She later visits Octavia in her tent, and laments the situation. Afterwards both say they can't move their forces, but then Morgan mentions that Ogam never mentioned anything about the two ladies themselves not being able to go, so they both sneak out and fight their way back towards Avalon.
Played straight near the end of episode 18, when Gaius has reached the last barricade in Avalon. Many of the characters trapped inside are genuinely scared that they may very well be killed once the Empire's troops breach the last door.
Oh Crap: Several characters display this look at times, usually when someone they're fighting turns out to be stronger than they are (usually when fighting Arawn).
Arawn himself has this look in episode 6 after he finds a seemingly discarded seal pelt, and gives it to Rhiannon to help her start a fire. Then it turns out that it was actually a magical MacGuffin for Llyr, without which she can't go back to her homeland any longer.
Even their God Watos isn't on their side anymore (Merkadis is defeated by a prayer that invokes an powerful ray of light from the heavens, and Lector is pretty much doomed when Watos doesn't answer his pleas for help). It is also hinted that the Angels do not know the true will of Watos and that Arawn/Lucifer was specifically created by Watos to free the world from his kin's control before they find another reason to trigger apocalypse.
The Other Darrin: There was a massive re-casting effort between the PC and PS3 versions, replacing most of the original cast with a more high-profile one (which carried over to the anime) To wit:
Precursors: Before the Humans of the Iron Age, there were the Elves of the Bronze Age; before the Elves, there were the Giants of the Silver Age; before the Giants there were the Dragons of the Golden Age. All three previous Ages met a tragic end no thanks to the Angels.
Rage Against the Heavens: Happens to Arawn, who hated how the spirits were abusing their powers with the mortals living on Earth, and causing destruction of the various ages simply because they couldn't live up to the angel's standards of perfection.
Real-Time Strategy: The core gameplay mechanic of the game, or to be more specific, semi-RTS combined with JRPG.
The PC game has it too for every super move. All four of them.
Surrounded by Idiots: Arawn feels this way. He later extends this by saying that humans are such idiots for obsessing over chivalry and honor at the expense of freedom — freedom he once sacrificed his own Angelhood for.
Suspiciously Specific Denial: Done by Arawn, of all people. "I-it's not like I'm doing something effeminate like wallowing in nostalgia or anything!"
To Be Lawful or Good: Arthur faces this dilemma often. He's keen on upholding the laws of his tribe (which are not always good and tend to advocate death as solution to every problem), but he's also a good person. Other characters are more willing to bend the rules if they are to stay alive, and halfway through the series Arawn convinces him to follow his conscience.
Tranquil Fury: Arawn is quite an adept swordsman, and often moves calmly and coolly when in combat.
Trojan Prisoner: Octavia tries this out with Morgan in order to sneak the two of them into Avalon, which is under siege by the Empire. It would have worked were it not for the fact that the unit she associates herself with is a unit that had been sent back to the mainland long ago. They get to fight their way inside instead.
The Unfavorite: Arawn to the other Angels. Likewise he feels the same hatred to his kin, as evidenced by his own face in a mural on his tomb being scratched out.
Tsurime Eyes: Lidia. This is also notable in flashbacks where she and Octavia were friends she didn't have these.
Unwanted Harem: A lot of girls follow Arawn around, whether he wants them to or not.
Video Will: Arawn leaves one of these in a prison located inside Mt. Corus, and a thousand years later, Taliesin is led to it by a baby dragon.
War Is Hell: Ogam points it out to Arthur when the two meet after Arthur runs off for attacking Arawn while under Lector's mind control.
What Measure Is a Mook?: Bubulcus in episode 6, after falling into an ambush set up by Arawn and Arthur, cuts the suspension bridge he and his men are escaping on to prevent the Gael's from pursuing them. However, several of his men, as well as Octavia, are still on it when he finishes cutting it, causing some of them to fall to their deaths. Then he later has the gall to get angry at Octavia when she reports what happened to his superiors.
You Can't Fight Fate: Taliesin spends his life running away from his prophesied fate as chief of Brigantes. In the end, he decides to embrace it in a Heroic Sacrifice... only to survive unscathed.
There's a little twist to this tale, as mentioned on the PC version of Taliesin's part of the ending: He is phropesied also to be killed by a terrible beast. He befriended a dragon cub.
You Killed My Father: In Episode 15, seeing Arawn use the power of Light reminded Arthur of the Angel who killed his father and calling himself "Arawn". In a fit of rage he tries to run him through even when Rhiannon gets in the way, forcing Arawn to shove her and take the sword to his gut. Only then does Arthur calm down and realize what he's done. He apparently forgot that Arawn was asleep for the last 1,000 years and couldn't possibly have killed his father. Then it turns out that he was brainwashed by the true killer — Lector.