Has shades of this in earlier books when her internal monologues show her being distracted by stuff or pointing out irrelevant details. Becomes more sinister in later books when you realize that the voice is Darquesse pointing things out to her.
Break the Haughty: Last Stand of Dead Men. First she finds out about Ghastly. Then Stephanie tries to kill her and drives her out of the house. Then she kills someone in a fight. Then Darquesse takes over, seemingly for good.
Freudian Trio: If you count the reflection as being a part of her then she her various personalities make her one by herself. Darquesse is the id, Stephanie is the superego and Valkyrie is the ego.
Foil: Is increasingly becoming one to Skulduggery. Whereas Skulduggery is working hard, partially thanks to her influence, to redeem himself and become a better person (and it seems to be working—contrast his actions in the first war to the one in The Last Stand of Dead Men), Valkyrie is taking several levels of jerkass, and seems to be attempting to emulate the person he was, rather than the man he's trying to be, as Fletcher points out. They're also foils in how they deal with their Super-Powered Evil Side. Whereas Skulduggery appears to be more or less in control of Lord Vile as of The Last Stand of Dead Men, in which he uses powerful Necromancy without his armour, and even before that took steps to lessen its influence, Valkyrie's alter-ego has taken complete control of her and refuses to change.
Hearing Voices: Probably not a good sign. They continue after Darquesse is gone.
Heroic BSOD: Has a minor breakdown in Last Stand of Dead Men after Stephanie tries to kill her and drives her out of the house, but she goes practically catatonic after killing a woman in a fight. Darquesse takes full advantage.
Hormone-Addled Teenager: Davina Marr accuses her of this and being smitten with Skulduggery in Dark Days, but it's more prominent from Death Bringer onward.
Ax-Crazy: While he seems perfectly normal (aside from the obvious lack of flesh and organs) a few people, including Skulduggery himself, have alluded to him becoming a bit of psycho after his family died. It started to show after Dark Days.
Don't try to manipulate Valkyrie, don't threaten Valkyrie, and for God's sake don't harm Valkyrie in any way. Best just leave her be. A few people have told Valkyrie that the only reason she isn't dead yet is that people know how Skulduggery will react. This ends up saving her life in Kingdom of the Wicked.
Foil: Is increasingly becoming one to Valkyrie. Whereas Skulduggery is working hard, partially thanks to her influence, to redeem himself and become a better person (and it seems to be working—contrast his actions in the first war to the one in The Last Stand of Dead Men), Valkyrie is taking several levels of jerkass, and seems to be attempting to emulate the person he was, rather than the man he's trying to be, as Fletcher points out. They're also foils in how they deal with their Super-Powered Evil Side. Whereas Skulduggery appears to be more or less in control of Lord Vile as of The Last Stand of Dead Men, in which he uses powerful Necromancy without his armour, and even before that took steps to lessen its influence, Valkyrie's alter-ego has taken complete control of her and refuses to change.
It went so badly that he switched sides with the sole motivation of being able to kill things. Mevolent's other generals believed that bring back the faceless ones was the correct thing to do. Vile wanted them back so he could try and kill a God
Names To Run Away From: Skulduggery Pleasant might not sound so bad but "The Skeleton Detective" is a little more ominous. Most sorcerers have heard of his reputation and would rather surrender than face him.
Roaring Rampage of Revenge: At first it seems like this was Skulduggery for a period of a few weeks after his family's death. He reveals in Death Bringer that it was several years before he managed to calm down. The entire time he spent as Lord Vile was basically this, with the whole world as his target.
Rousing Speech: Used to be quite good at these apparently. Not so much anymore, though he still has his moments.
Sad Clown: Skulduggery does love his wisecracks, but is also one of the most screwed up characters in the series. Some of his jokes tend to be rather dark. His "fun for the whole family" remark after Serpine decides to torture him comes to mind.
Would Hurt a Child: He attempts to shoot Kitana out of necessity more than anything else in Kingdom of the Wicked. He is, however, unwilling to use lethal force on the young recruits that the American Sancturary has blocking their path in Last Stand of Dead Men.
Crazy-Prepared: Her magic is based on sigils which she has carved into her apartment, her furniture, her library, tattooed all over her body (and even on her horse) which range from creating energy beams to making her stronger or even throwing all the furniture at her attacker. When Skulduggery came back, Mevolent had her carve one onto her torso that would ensure that he would really stay dead.
Heroic Sacrifice: Subverted in Last Stand of Dead Men. China uses a particular spell devised by Mevolent, of all people, to burn herself alive in an attempt to take down Madame Mist, which succeeds. However, even after a Dying Declaration of Love to Skulduggery, she survives thanks to Darquesse's intervention.
The Vamp: Has her moments. May have been one before she turned "neutral."
We Used to Be Friends: As of Death Bringer, Skulduggery is seriously pissed off with her to say the least. Valkyrie's not too thrilled with her either. She's amicable with Valkyrie by Last Stand of Dead Men and seems to have reached some kind of understanding with Skulduggery, even if he can't forgive her.
Book Ends: Combined with Mythology Gag. She was supposed to have been killed by the White Cleaver in the first book. Derek Landy was told that too many people were dying; so she was given a different fate instead. Finally, in the last book; she faces what it turned into, the Black Cleaver; and comments that she may have originally been destined to die during their first fight.
Bi the Way: The only 'romances' involving her that we see are with Ghastly and Sanguine, but she also casually mentions having once had "a bit of a thing" with Aurora.
Cold-Blooded Torture: Frequently the victim of this because the author wasn't allowed to kill her off in the first book.
Wide-Eyed Idealist: Has shades of this in the first book. She feels really guilty about sending Cleavers to their deaths to save Skulduggery, something that Skulduggery and Ghastly have come to accept.
Dating Catwoman: So far his love interests include Darquesse (the harbinger of the apocalypse), Myra (an Australian assassin who is trying to kill him), and Stephanie (a murderous Ambiguously Evil reflection). Dude cannot catch a break.
Ambiguously Evil: Gets steadily creepier as the series progresses, and ultimately murders Carol and tries to kill Valkyrie. But in Last Stand of Dead Men she gets a Sympathetic P.O.V., starts fighting to protect her family and is clearly smitten with Fletcher.
Valkyrie starts treating it a lot better in Kingdom of the Wicked out of loneliness. She's outright stated that she considers it a person now. Unfortunately, it agrees.
Reflection: I'm not a thing. I'm not an it. I'm a person. I'm a her. I have a family who loves me. I have parents and a little sister who I'm going to take care of. I have friends in school, and I want to go to college. I want a normal life. A happy life. A life. And I'm going to take it.
Asshole Victim: Neither his extensive torture by Darquesse or being thrown into Accelerator by Skulduggery get him much sympathy after he killed Ghastly and Shudder.
Big Bad Friend: Orchestrated everything bad that happens in Last Stand of Dead Men, except for the attack on Roarhaven and Darquesse's arrival (which were both still his fault, but less intentionally).
Faux Affably Evil: Re-read the part where he's having his little moment with Skulduggery. If it weren't for the fact that Skulduggery was chained to a chair, it almost would seem like they really are, as Skulduggery puts it, "two guys chatting."
Hoist by His Own Petard: Doubles with Exact Words: By turning himself undead, the Sceptre of the Ancients (which cannot be used unless the last owner is dead) will respond to him, and anyone else who uses it, and Skulduggery is able to use it to kill him permanently.
Squishy Wizard: Has some very formidable, very scary powers that he's all too happy to use on people...and then he goes and gets his ass kicked by Skulduggery whenever forced into close-combat. Repeatedly.
The Starscream: In the alternate universe. betrays both Mevolent and China
Enemy Without: Eventually she's forced out of Valkyrie and possesses Stephanie's corpse instead.
Even Evil Has Loved Ones: Darquesse gets pretty pissed off with Ravel for murdering Ghastly and goes out of her way to punish him in The Dying of the Light. She's also in love with Skulduggery. This does not stop her from trying to maim/kill him, but she does feel bad afterwards.
Good Thing You Can Heal: It's a good thing that Darquesse can heal otherwise her fights with Lord Vile and Mevolent would have been remarkably one sided.
How Much More Can She Take?: Lord Vile slits Darquesse's throat, shoves his thumb in her eye and generally delivers a No-Holds-Barred Beatdown that leaves her on the brink of death. She pretty much walks it off. On one hand... yay! Skulduggery didn't kill Valkyrie! On the other hand... Darquesse. Just. Doesn't. Die.
In Kingdom of the Wicked she survives her heart being crushed, her jaw being burnt off and beheading.
Darquesse is prophesied to be this, but has mainly restricted herself to tearing apart people who are trying to kill her or in her way. She outright stated that she doesn't want to destroy the world because "it's funny.".
She goes all the in Dying of the Light. Thankfully, it's a trick.
One-Man Army: Darquesse literally takes out an entire army in Last Stand of Dead Men.
Pet the Dog: Darquesse saves China after the latter hesitates in trying to kill her.
Back from the Dead: Sanguine kills him in Dark Days after finding out he's not a killer, but Scarab decides they may as well put his death to good use, and makes him a zombie. Then in Kingdom of the Wicked he gets Dr Nye to bring him completely back to life. See Gender Bender.
Took a Level in Badass: After spending the last few books magically crippled thanks to Valkyrie, he is in full Badass mode in The Maleficent Seven.
Tunnel King: His main power note Well, his only power, unless you include cutting people with a razor is being able to tunnel through walls and the ground. He can also grab you, pull you into the ground and leave you there so you're stuck.
The Worf Effect: He's a completely unstoppable badass, who lasts all of five seconds against Lord Vile before he's ripped apart.Similarly, the Black Cleaver beats the crap out of Skulduggery until he uses Necromancy.
Friendly Enemy: Seems to be on these terms with Valkyrie after the operation.
Graceful Loser: After operating on Valkyrie during life-threatening circumstances, it seems to take it rather well considering the fact that it lost an opportunity to look for a soul (And y'know, the fact that it threatened to kill her).
OOC Is Serious Business: Nye spends most of its time in Mortal Coil being condescending, sick and twisted, and is not at all intimidated by Valkyrie, even when she's alive with her magic. When she is dead with necromancer magic, on the other hand, it's practically wetting itself with fear. This is a hint that Lord Vile is also dead
Shout-Out: When it dissects the conscious Valkyrie on the operating table, it's very reminiscent of Claire Bennet's trials and tribulations in Heroes.
The Heavy: For much of Last Stand of Dead Men Bisahalani's the guy calling the shots on the opposing side, and the Man with the Golden Eyes is the Big Bad, but Mantis is the one leading the Mooks, so it's the most direct antagonist.
Heel-Face Turn: Once the Sanctuaries stop fighting each other and start fighting the Warlocks.