In anyone else, these could be called out as Blatant Lies. But what sets this character apart is that, contrary to all evidence and the fact that they, by all rights, should know better, they honestly believe every word they're saying.
However sane they may have been when they started, they've gone off the deep end and are now Believing Their Own Lies.
Double Think is an extreme example where the said liar does know better but keeps believing his own lies simply because he can. Sister Trope to A God Am I, where there is frequently overlap. The key difference is that this trope is less specific and doesn't have to be a claim of Godhood. This trope also applies only when the character should know perfectly well they aren't a god, but have convinced themselves otherwise. Characters suffering from this trope are also prone to a Self-Serving Memory.
See also Becoming the Mask, in which a character assumes a fake identity he ultimately wishes to keep; and the Amnesiac Liar, who gets fed their own lies after memory loss. A Straw Hypocrite, who manipulates others by feigning to follow a cause, may get taken in by their own rhetoric this way. Compare Conspiracy Theorists, who think their outrageous claims are true from the get-go. With a little Obfuscating Stupidity, one can pretend to believe for as long as this gives an advantage.
Compare Getting High on Their Own Supply, when a person who peddles illicit substances becomes an addict themselves; while the tropes are separate, the phrase is often used as a metaphor for this one.
- In an Archie Comics story, Veronica tells Archie and Reggie that whoever scores the most baskets in the next school game gets to take her out that weekend. Reggie attempts to sabotage Archie by telling him that the best way to improve his basket-shooting is to criticize himself constantly and harshly while practicing. This goes Reggie's way until Coach Clayton sets Archie straight, telling him that he should do the opposite while practicing and build up his confidence. Archie indeed goes on to score the most baskets, leading Reggie to wonder whether there was something to his "advice" after all. The story ends with Reggie practicing while berating himself and surrounding himself with demotivational posters.
- Avatar The Last: Airbender - Rebound: Mai's father sincerely seems to have deluded himself into believing Fire Lord Zuko abandoned his daughter and kicked her out when it's common knowledge that Mai is the one who left Zuko.
- Batman: The Joker sometimes believes his Multiple-Choice Past, Depending on the Writer of course. One issue of Robin (1993) has him actually in tears as he tells the psychiatrist of his abusive childhood, only for the psychiatrist to coldly point out that it's the seventh story he's told now.
- The Flash: This played a crucial role during "The Return of Barry Allen" and is explained in full after the man everyone thinks is the revived Barry turns out to be Professor Zoom. As it turns out, following his Laser-Guided Amnesia from the trauma of time-travel and seeing a museum exhibit depicting his own death at Flash's handsnote , he mentally convinced himself that he was Barry, and this helped him to successfully pass the probing of Hal Jordan's ring, which would otherwise have outed him as a fraud right away if he hadn't honestly believed it to be true.
- The Smurfs: In The Betrayal of Smurfblossom, Smurfstorm falsely claims that Blossom has a crush on Hefty in order to discredit her after she suggests that Storm should accept that they tied in a contest. Over time, however, her own secret crush on Hefty makes her start to suspect that she might have been accurate, making her increasingly jealous.
- Judas Traveller from Spider-Man was a guy with apparently limitless Reality Warping powers who claimed to be a near-godlike immortal sorcerer. It eventually turned out he was a former psychologist with a psychic ability to make people believe things... himself included.
- Superman: Depending on the Writer, sometimes Lex Luthor actually believes that he is fighting to protect humanity from Superman. Other more minor villains, like (the most recent version of) Sam Lane, may believe the same.
- A recurring theme in IDW Publishing's Transformers comics:
- Onyx Prime was initially a brave warrior and crusader who could bring the rest of the Thirteen Primes down from their storytelling and blind idealism. Then Alpha Trion pointed out to him how stories could be used to inspire people; Onyx decided to try that out and began making up stories about himself, the other Thirteen, and their allies. At some point he started believing his own propaganda and devolved into a total egomaniac, partly leading to the first major war on Cybertron.
- Alpha Trion later argues that all of the Thirteen did this and he has a good point. We're shown that Nexus Prime died because when Galvatron came to kill him, he responded by spouting nonsense about how "only a Prime can kill a Prime". It was blatantly untrue, but Nexus seemed to genuinely think he was immortal, not bothering to defend himself or flee at all. Galvatron killed him with one shot.
- Most of the Functionist Council enforce their oppressive worldview mainly to keep themselves in power, but Six-Of-Twelve eventually started buying into his own hype. He not only spouts Functionist rhetoric far more often and seriously than his comrades, but he continually claims to get visions from Primus (we never learn whether he's really having them or is just delusional) and fuses a fake Matrix into his body, utterly convinced that it's the real deal when even a casual observer like Drift can tell it's just a replica.
- Unknown Soldier: One iteration of the Unknown Soldier suffered a mental breakdown after liberating a Nazi concentration camp. Now he believes that whatever America does is right, no matter how horrible, because they once fought against the horrible Nazis.
- Walter, ultimately in Big Eyes. He insists he's an artist and continues to do so until his death, even after Margaret uncovers signatures on his paintings that prove they were really just mass-produced tourist souvenirs from France.
- Marcy seems genuinely surprised by the rashes she finds on her back, shortly after assuring Paul that she was perfectly healthy in Cabin Fever. Even after seeing the rashes, she seems to convince herself that they are just marks left from when he grabbed her.
- In Dawn of the Planet of the Apes, Koba claims that he fights for his fellow apes, and the conviction in his voice heavily implies he may very well believe that, but his actions in betraying and trying to gun down Caesar and callously killing any apes who disobey his orders or get in his way, reveals the real Koba: a scared, desperate, angry, and selfish individual who acted out of sheer hatred for humans than for the well-being of his people.
- In Dead Man Walking Matthew Poncelet has convinced himself that he didn't rape a girl and then brutally murder her and her boyfriend. He holds firm to this claim for a large part of the film. However, towards the end when he is pleading against his sentence to the death penalty he breaks down and admits that he did, in fact, commit the crime. While this could be seen as him admitting what he already knew, it is far more likely that he purposely suppressed those memories and began to believe his own lies. Thankfully he redeems himself at the end.
- Downfall: Despite giving orders of Germany's self-destruction, Hitler insists to people around him that the war could be won with miracle weapons and reserved troops somewhere. This would have been Blatant Lies on Hitler's part, but Hitler genuinely believes in them as his infamous Villainous Breakdown is the result of learning that Stiener is unwilling to intervene due to lack of soldiers.
- In My Country: Colonel De Jager and Boetie both tortured and brutally killed many black civilians, but insist that they were only protecting their country from terrorists.
- The Informant!: The Reveal of the film is that the main character is a Compulsive Liar with an extraordinary talent for self-deception.
- Pain and Gain: Danny Lugo, a con artist, a thief and a kidnapper who only impersonates a businessman, beats up Frank Griga, a wealthy phone sex operator for first refusing to invest in his "business" and then insulting his lack of experience in investment pointing out Lugo sounds like an amateur who has little working knowledge of the financial market. Lugo's line after the first punch against Griga is, "NO ONE CALLS ME A FUCKING AMATEUR!"
- John Kramer, better known as Jigsaw, takes people he considers unworthy or unappreciative of life and puts them into death traps as a means of rehabilitating them. Despite all evidence to the contrary, he's not only convinced that his methods are effective, but that he's not really a Serial Killer because he's never intentionally tried to kill anyone. To him, people who die in his traps lacked a survival instinct. In fact, John is so adamantly against murder that when someone built their own trap to kill a criminal and framed him for it, John personally kidnapped him to lecture him on his methods and try to convince him that his are more effective. At his core, his intentions really are noble but he deludes himself about how noble they are.
- In The Silver Chalice, Simon the Magician (Jack Palance) is a conman who gets rich by faking miracles. He convinces Caesar that he is able to fly, but eventually comes to believe in his own magic, jumps off a tower, and plummets to his death.
- Step Up 3: Julian was kicked out of the House of Pirates for throwing a competition over a bet, but when he insists to his sister that they kicked him out because Luke was jealous of him, he seems to genuinely believe it.
- Exalted: Desus honestly believes his own Villain with Good Publicity reputation, because he's affected by the same mind-manipulating magic that forces everyone else to rationalize his horrible actions away.
- Forgotten Realms: Cyric, the God of Lies, guided a mortal author to write a book called the Cyrinishad which would make anyone who reads it or hears it read aloud believe that Cyric was the greatest of all gods. Cyric then read it himself and fell victim to its enchantments, bringing his megalomania to new heights. For a time, he saw his enemies as too "insignificant" to care and even got Madness in his portfolio. Later it was discovered that the only way to get rid of this for a deity involves a drop in Divine Ranks.
- Genius: The Transgression: The Phenomenologists can spend Mania to immediately achieve an Exceptional Success on any Subterfuge roll. This is because they literally never think they're lying — they always believe exactly what they're saying. And when they change their mind, they'll believe the new thought just as strongly.
- In Nomine: The Balseraph demons essentially have this as the core aspect of their character. As fallen Angels of Truth, they become Demons of Deception, capable of weaving lies that others end up believing without question. But to do this, a Balseraph must first convince himself of the lie, warping his own personal truth to reflect the lie. For example, a Balseraph trying to convince a bar bouncer that he's a VIP must first convince himself that, "Yes, I'm a VIP, and I've been at this club dozens of times. Why isn't that bouncer letting me in already?"
- Promethean: The Created: This is one of the primary dangers faced by Prometheans pursuing the Refinement of Gold. This Refinement involves the Promethean living among humans and pretending to be one, but the Promethean must never forget that this is a pretense, that it is still a monster and that sooner or later the mask will slip and the torches and pitchforks come out.
- Ravenloft: This is the main flaw of Yagno Petrovna, Darklord of G'henna. He is the high priest of Zhakata, an evil god of famine. He stubbornly refuses to realize that Zhakata doesn't exist even though he made up almost everything about Zhakata himself.
- Rifts: The Coalition States High Command likes to blame all the ills in the world on magic and non-humans. In the beginning, it was just a convenient Scapegoat Emperor Prosek and his advisers used in order to grab more power. However, they've been telling their people the Big Lie for so long that they now believe it themselves.
- World Tree (RPG): Bonstables are pathological liars and deceivers, but never say anything that they truly think is false. Rather, they constantly delude themselves into thinking that whatever ruse they're keeping up is the actual truth — when they take the shape of a Prime species, for instance, they convince themselves that they are actually a member of that species for all purposes — and when they inevitably change their minds and start another deception rationalize things to believe themselves to have always been correct and honest at any given time. This makes them very difficult to discover, since mind-reading magic can't generally penetrate their delusions and lie detection reads them as perfectly honest and forthright.
- Celestia Ludenberg in Danganronpa: Trigger Happy Havoc, also known as 'Queen of Liars' and 'Ultimate Gambler', is capable of lying in a way that she could even believe own lies. Or so she claims. However, based on her free time events, we could probably guess that she probably also LIED on her identity and history, which are based on highly daring and impossible events you can only find in fictions, and her real name was a much more common 'Taeko Yasuhiro'. She got so over in her lies that she believed that she really is born as 'Celestia Ludenberg'.
- RWBY: Raven Branwen tends to justify her actions to herself using the same logic she uses to justify it to others, despite any misgivings she has. She tries her hardest to convince everyone, including herself, that she's acting out of a Might Makes Right attitude, despite everyone else seeing that she's more of a Dirty Coward. For example, Lionheart asks her who she's trying to convince when she's telling him that there's no shame or fear in working with Salem because she knows Salem can't be defeated. When Yang confronts her at the Vault about killing the previous Spring Maiden, she seems to be trying to convince herself it was truly a Mercy Kill as much as she is trying to convince her daughter. Yang brutally tearing apart this mindest causes Raven to have a Villainous Breakdown.
- Immortals in El Goonish Shive are bound by their word. If they make a vow and knowingly break it, they're plagued by constant mental reminders of their oathbreaking until it's rectified. Loophole Abuse is one way around this, but only if the Immortal himself sincerely believes the loophole is not breaking their word. When an Immortal declares that the best thing he can do in the name of his vow to one of the protagonists is absolutely nothing, he waits to see if there's any effect before being glad he believes his own hype.
- In Katamari, Ace is completely convinced that the Prince is a prideful, selfish Attention Whore who'll do whatever it takes to come out on top. In other words, he thinks the Prince is just like him.
- In one of the Las Lindas side-stories, a young tribal girl tries to set up an outsider as a god so that she can leave her island. And then the gophers that Minos threw with his super-strength start falling from the sky. She ends up getting a job at Las Lindas a few months later to follow Idward.
- The Order of the Stick:
- This was one of the critical flaws of Miko Miyazaki. She was fundamentally incapable of seeing herself in the wrong, and would frequently convince herself of whatever she had to in order to keep it that way even after the gods themselves strip her of all of her powers as a paladin in response to her killing of Lord Shojo.
- Additionally, Ian Starshine (Haley's father) raised his little girl in a Wretched Hive and taught her to lie at every opportunity whenever asked about herself. He's so paranoid he's incapable of taking people at their word and will invent elaborate scenarios which justify his ridiculous position and seems to totally believe them.
- In the furry comic, Tina's Story, Tina's stepfather, Stan, confesses to her that He is her biological father. The story Tina had been told her whole life - that her mother had been raped by a human - was a lie they told to cover the fact that Missy, Tina's mother, was underage when she became pregnant. Missy eventually started accepting that lie as truth and developed a deep hatred of humans as a result. Stan confesses this because Tina is engaged to and pregnant by a human male, and Stan didn't want that lie hanging over Tina's marriage or her children's lives.
- Unsounded: While initially Duane went along with his younger plats' claims that Duane still hadn't "lost a lad" after Jon's death to appease and keep their spirits up over time he started believing it himself. This is part of why reexamining the memory with his new mind which won't let him gloss over things and let things fade into the fog of memory like a human mind sets him spiraling.