When a hell-bell rings, a hellspawn gets its wings.
"No day goes by where I am not tempted to return to my inborn nature. Zin krif haalvut se sulyek.* Approximately, "honor fights the touch of power." What is better? To be born good, or to overcome your evil nature through great effort?"
it's not uncommon for there to be a mentor who has done this before and is willing to sponsor them, or more tragically, failed and wants them to succeed as a redemption by proxy. If they really are the first to pull it off, an author might invert the Last of His Kind and have the Ascended Demon redeem some or all of their peers and reveal they were Good All Along. This is especially poignant if the demons were once angels who fell. Expect an Enemy Civil War to follow as demon-kind is divided between "loyalists" and "redemptionists". In fact, just check out the Fallen Angel page; it has far more examples than this one does.
It's not all rainbows and lollipops though. Aside from the fear of backsliding into evil, the Ascended Demon has made enemies of his former kind*
DX-4, when he was young killed Bad End Generals because he needed the magical girls to commit a massive invasion on Gundalia.
In the first appearance of Necron in Green Lantern, one of his minions demonstrates great courage and disagrees with his boss' invasion of the living universe. He "ascends" to become a Green Lantern that's free of Necron's influence.
In The Deaths of Ian Stone the Harvesters aren't really evil per se... but being extra-dimensional spirits that live off fear, and have long ago become addicted to the fear of a human facing death (and the easiest way to get that is to kill people), they are pretty close to Always Chaotic Evil. However a renegade Harvester has found an alternate power source capable of killing the other evil Harvesters... Love.
While operating as a sort of Noble Demon to begin with, Extirpon in The Gerosha Chronicles views himself as an ascended demon when he unlocks the other half of his Marlquaanite abilities - the ability to heal others when charging up positive energy. With negative emotional energy from others, he was only able to heal himself, and otherwise had only the power to destroy, humiliate, and undermine. His blue-purple glow effects switch over to glowing white and gold when channeling his "positive battery."
In Tolkien's The Silmarillion, the Maia (angel) Ossė had thrown in his lot with Melkor, but eventually repented and returned to the good side. The book doesn't make clear just "how far" he fell, since he was not one of those who fell right at the beginning during the Ainulindalė. Nevertheless, he was fighting with the demons against the angels for a while there. Though in most ways he tried to make Arda 100% compatible with Catholicism, Tolkien explicitly disagreed with the idea that anyone could be truly irredemable.
In the Young Wizards series, the Lone Power Itself eventually gets redeemed... but since as one of the Powers That Be It's outside time, Its evil iterations continue to exist and cause trouble in the series even after Its redemption.
A variation in The Dresden Files: Harry briefly touches and rejects a coin possessed by the Fallen Angel Lasciel, who cannot fully possess him but leaves a "shadow" in his mind to try and corrupt him into fully accepting the coin. However, while Lasciel herself is eternal, unchangeable and irredeemable, the sentient image that is left in Harry's mind is only as unchanging as the mortal brain she inhabits, and over the course of several years she changes and develops a personality of her own. She eventually comes to care for Harry so much that she performs a Heroic Sacrifice with a psychic version of Taking the Bullet. This has never happened before because all previous shadows have managed to tempt their hosts into fully accept the coin in such a short time that the shadow has no chance to change, but Harry is too much of a Determinator to be broken that easily.
This is the attitude of any warlock (a practitioner of Black Magic) who tries to become a wizard (a practitioner of good magic); Black Magic gets you Drunk on the Dark Side extremely easily, and even if the warlock has a My God, What Have I Done? and tries to reform, it's all too easy to slip back into it. As such, very few warlocks are able to reform, and while this is partly because the White Council executes anyone (even teenagers who genuinely don't know what they're doing) who performs Black Magic unless a member of the Council puts his own life on the line to mentor the warlock into becoming a proper wizard, it is suggested that reformed warlocks would be pretty rare even without the summary executions. Harry Dresden himself is one such reformed warlock, and even though he only performed Black Magic once in self defence when he was a kid, he fully recognises the ease of recidivism and, much as he despises the Council's arrogance and self-righteousness, he has to concede that in the majority of cases their policy is probably necessary. However, when he truly believes the accidental warlock Molly Carpenter has a chance of reforming, he fights tooth and nail to give her that chance.
Reaper had a group of demons who were trying to overthrow Lucifer through being nice. When they decided to stop playing the long game and just kill the devil, Steve was the one dissenter who pointed out that that defeated the entire point. When the devil shows up and kills the entire army, Steve apparently ascended to angelhood after death, though the finale indicates that angels can be pretty ruthless when they need to be.
Mythology and Religion
The 17th century Lemegeton and 19th century Dictionnaire Infernal (both demonology books) depict Marchosias, a literal Noble Demon looking to achieve this trope.
On the other hand, islamic traditions have it that djinn can be redeemed (though it helps they're only partly demon analogue). In Judaism, fallen angels are not necessarily evil, and many still are considered part of the heavenly host.
Meridiana was a succubus believed to help Sylvester the second become the Pope. This was meant as slander most likely but since his time as head of the Catholic Church is best known for cracking down on Simony, promoting scientific studies, building musical instruments such as a hydrological organ and teaching grammar his fans concluded that if Meridiana helped put him in power she was a benevolent succubus.
The Dungeons & Dragons sourcebook "Book of Exalted Deeds" on good-aligned characters has one chapter on forgiveness. It features an orc chieftain sparing a girl's village because she had saved his life years earlier and a good-aligned illithid.
One of the (Many) nicknames for Planescape's Lady of Pain is "The fiend of blades", started by someone with the theory she is an ascended fiend. Given that he wasn't flayed (like people who give... less than flattering nicknames) and lived long enough to die of natural causes, this indicates there isn't really an issue with the term.
In Pages of Pain she remembered being a daughter of Akadi and Poseidon and Set's fiance. That is, Lady is a born Power, has the Pain sphere/domain, as a gift and would be a goddess if she didn't refuse to.
There was a chant about the agathinon Janarr and erinyes Nalura working in mortal disguise on the Prime. They sort of just did their jobs—she tried to seduce him, he tried to turn her attitude up to celestial standards. Before they knew whom exactly they faced, both succeeded. They had to hide from their respective bosses from that point on, though.
K'rand Vahlix is a general of the Risen Fiends who have fled to the various Upper Planes, organizing them into the Celestial Hosts, and is so powerful and good-aligned that he is completely unafraid of any Deep Cover Agents that might assassinate him, which is the main obstacle to most Risen Fiends associating with each other.
In Legend Of The Five Rings the demon Okura no Oni affected the honor of the Lion Clan through Kitsu Okura, and at Oblivion's Gate slew her father, Akuma no Oni. She ascended to become the Guardian of Tengoku and she is now seen guarding the Gates of the Celestial Heavens.
In Vampire The Masquerade the fabled state of Golconda allows a vampire to transcend just about every physical and spiritual weakness of vampirism. The Enemy Within is muzzled, blood requirement drops to almost nothing, and various caps and restrictions vanish. Tellingly, the vampire is immune to Diablerie... their soul is no longer tied to their blood/body and can't be eaten.
In Vampire the Requiem the above is one of three suggested outcomes for reaching Golconda. The best one is a return to humanity. The worst one? Complete balance with the Enemy Within... which means the vampire is in perfect harmony with a remorseless gluttonous raging monster.
Another World of Darkness gameline with this possibility is Demon: the Fallen. The titular demons have the choice to either try to recover their lost grace by reaching zero torment (only possible with RP, no game system allowed that) or spurning their newfound humanity and chance at redemption by Slowly Slipping Into Evil. Unlike Vampire, Demons who eschew use of their demonic powers and One-Winged Angel form are at a disadvantage compared to their morally laxer peers.
In the Mystic China RPG from Palladium games there's a literal Ascended Demon class, where as you gain levels you lose demonic traits and gain human traits until if one survives all the pitfalls the character will be reborn as a human being with only dim memories of its former life.
The Abyssals are Exalted by the Neverborn, who corrupted Solar shards to serve death and the Void. Thing is, underneath all that necrotic taint, there's still a glimmer of innate Solar nature. It may very well be possible for an Abyssal to seek redemption and become a Solar again... and if that happens, then they don't have to deal with that peskyGreat Curse.
The Infernals are similarly corrupted Solars, only their shards were tinkered with by the Yozis as part of a scheme to escape from Malfeas by making Creation just as bad. Infernals don't have innate Charms as other Exalts do, but instead have a direct connection to their Yozi patrons that allows them to use their magic. There's just one problem that the Yozis didn't entirely consider — that relationship may go both ways. So, with enough time and effort, an Infernal could possibly convert one of the demonic masters of Creation to good.
Alternatively, Infernals using the right Heretic Charms can redesign themselves into Primordials 2.0, taking and improving the designs that the Yozis used before being ripped inside out, castrated, and imprisoned. The resulting being literallycontaining worlds and having multiple souls. It doesn't require the character to be good, or even define good, but it does involve long periods of spiritual meditation and reformatting one's own souls, and it's hard to get worse than being one fifth Ebon Dragon, the Cosmic Principle of Dickishness.
Sidereals with access to Greater Astrology Charms actually have one, An End to Darkness, that allows them to hand this out willy-nilly. If they find a demon that wants to leave the service of Hell, they can cast this Charm and transform them into a god that will serve Heaven. The only downsides are that other demons will be out for that god's blood, and the Sidereal takes a large amount of Paradox in the process.
For that matter, your character can end up as one too.
Due to the number of times Aribeth passes through the Heel Face Revolving Door, some of her later incarnations come across this way.
Persey in the Tales of Arterra mod discovers a conscience and develops into a priestess of the goddess of love by the end of the story.
The player character in Dark Messiah can end up this way with one of the good-aligned endings. You lose your ability to transform into a demon and your succubus advisor, but you gain a bunch of top-tier holy gear after purifying yourself.
Similar to the Succubus Paladin in DnD above, in the MMORPG Mabinogi, the NPC Kristell was once a succubus, but after falling in love with the druid Tarlach, she became the priestess at Dunbarton.
Ashrah from Mortal Kombat is a demon, formerly from the Legions of Hell, who gets betrayed and has a Heel Face Turn in regards to Black and White Morality. She comes across a sword that purifies her, every time she slays a demon with it. She becomes a Woman in White. Her plan is, to gain enough purification, that the evil or taunt, that resided in her demonic creation, no longer exists, to the point of the Netherrealm (Hell) being UNABLE to keep her presence inside, for you cannot exist in that realm without a taint on the soul. So she'll eventually be expelled out of that fiery wasteland. The problem though? She's turned a bit Knight Templar, or Ax Crazy because of this obsession.
Granted, it's still unclear whether any of this is actually happening, or if the Kris is just a Manipulative Bastard of an Empathic Weapon that's making her think killing certain targets will save her soul in order to make her into its instrument of genocide.
Her ending in one of the games reveal that the Kris really works, but the rate of purification is proportional to the evilness and power of the target. Also, once the user is ascended (and safely expelled from the Netherrealm), the sword stays behind to repeat the process for another demon.
Worlds In Global Battle Locked, the sequel to Nexus War, has Redeemed demons and Fallen angels as third-tier classes. The Redeemed's skillset has a strong emphasis on protection and debuffing over dealing damage.
Planescape: Torment has Fall-From-Grace, a Succubus who runs a pleasant brothel of platonic prostitution. (A chaste succubus, who runs a brothel that does not sell sex, something that seems a contradiction in every way.) However, she still feels some loss from her ascension to a good-leaning Lawful Neutral cleric from a Chaotic Evil existence, hence the name.
There is evidence that her "fall" is more than a little uncomfortable for her, as her race is evil incarnate in a universe where belief governs the laws of physics and 'being evil' is as much a part of their existence as 'breathing air' is for humans. Despite your party being composed of only those in true torment, Ravel claims that Grace suffers more than any of your companions. This is pretty amazing, considering how your remaining party members are tormented.
A Satyr in World of Warcraft who came to his senses and seeks redemption culimnates when he uses parts of his former tribe and rips out his own heart to use in a potion to save a child dying of sickness. He doesn't actually ascend though: He just turns back into a night elf.
The Elder Scrolls V Skyrim: Paarthurnax, leader of the Greybeards, views himself as such; dragons are inherently born to conquer and dominate, and he supresses this desire only through meditation and great effort. Though the Blades doubt this, and it's hinted he might not be quite as good as he seems.
Flonne has become this, as of Disgaea 4. That is to say, she was originally an angel, but in the Good (and canonical) ending of the first game, the Seraph turned her into a Fallen Angel. Not much of a punishment (or even intended to be one), since she gets to stay with her friends Laharl and Etna, and her personality remains the same. By D4 she has apparently gotten a promotion, and his now an Archangel, once again serving the Seraph (and is the boss of the new Vulcanus).
Cthulhu in Cthulhu Saves The World was initially forced into becoming this, setting out to save the world so he could later conquer it. However by the end of the game he's grown to like being a good guy.
In Persona 4 and its spinoffs, Teddie, who was formerly a Shadow just like most of the enemies in the game, gains a Persona and essentially becomes human.
This also applies to the Personas of the main cast, including all of Yu's Personas besides Izanagi: all of them were originally shadows that were brought under the control of the ego.Persona 4 Arena implies that this process is reversible, and that it is only through sustained effort that a Persona can continue to exist.
The backstory of the Devil May Cry series has the "Legendary Dark Knight" Sparda rebel against his kind and fight for humanity, eventually fathering twin sons Dante and Vergil with a human woman named Eva.
Caliban of Narbonic, who renounces his demonic powers and becomes human. Turns out he's less motivated by virtue than by getting away from some demonic loan sharks who are after him, but once that's sorted out he seems quite happy to live as a human and gets a job at Starbucks.