In fiction, white locks on a young character mark them as mystical and arcane, whether it means they are outright magical or not. In sci-fi and fantasy, it's often the only non-human hair color you will see on new species or race of people, especially in elves or Human Aliens. It is a common mark of a Mysterious Waif and a Mystical Waif .
The popularity of this trope probably stems from its status as a borderline-natural hair color: white hair on a young person strikes that delicate balance of being unusual but not blatantly unrealistic. The color also lends itself well towards symbolism.
It bears repeating that these are young (or at least young-looking) characters, and their hair color is not indicative of age (or of premature aging).
See also White Hair, Black Heart, which is where having white hair is a sign of evil instead of magic, superpowers or otherworldly origins. If a character's hair turns white due to a disease or illness, it's Disease Bleach. If a characters hair turns white when they use their powers, it's Power Dyes Your Hair.
The eponymous Index has silvery-white hair, a Photographic Memory, magical abilities, plus information from 103,000 grimoires hardwired onto her brain. The setting takes place in a city dedicated to the study of people with psychic powers.
The character "Accelerator" is albino. He's a rank 5 Esper (psychic) who is also described as He Who Wields the Power of a God, for his ability to redirect any psychic ability thrown at him with telekinesis. To be entirely specific, his power is the ability to redirect vectors... which are present on nearly every aspect of the physical world.
Meiko "Menma" Honma from Anohana: The Flower We Saw That Day. The symbolism works both ways, as after dying as a child, she has grown, but her demeamor remains the same, so she's a ghost but also innocent.
Also by CLAMP, in Clover, Suu has white hair and is the most powerful magical being in that world.
Shiro from Deadman Wonderland is a Heroic Albino (well, when she isn't acting evil or upon her Yandere tendencies) with white hair and red eyes. The word "Shiro" in Japanese literally translates to White in English.
Fairy Tail has Mirajane, Elfman, and Lisanna Strauss, sibling wizards.
Ginko's white hair, missing eye, and green eye indicate being marked by mushi, which attract bad mushi to him and force him to live a life of traveling so he doesn't unintentionally harm people. it's later revealed the mushi that affected him was called "Ginko", which he took as a name after it took all his memories of his life before. It was the only name he remembered at the time.
The chapter "String from the Sky" revolves around a woman named Fuuki whose hair is white indicating she can see mushi. During the chapter she is affected by a mushi called Tenpengusa, which lifts her up to the sky and drops her leaving her between the human world and the mushi world. The chapter is spent on Ginko helping her become visible to people close to her, and returning to the human world again.
In the final chapter of the manga Ginko meets a spirit of the mountain born as a human girl. Given that the spirits of the mountain's job is to keep the ecosystem and living things on the mountain in balance she was marked by ancient mushi and was born with white hair.
The original Queen Serenity in Sailor Moon. Sailor Moon herself was originally supposed to be white haired when transformed and blonde as Usagi. This was vetoed by Naoko's editor Osa-P. In the manga, she does have white hair as Princess Serenity and Neo Queen Serenity, but not in the anime.
Rally Cheyenne from Silent Möbius is half-Lucifer Hawk (demon, basically), and her silver hair marks her as having some of the strongest magic in the series.
An interesting variation occurs in the Slayers series: protagonist Lina has reddish colored hair, but when she casts a particular high-level spell, her hair turns white. This occurs once in the Light Novel series, and twice in the anime.
A straighter example is Millina, a white haired female mercenary and Lina's ally from the second half of the novels. However, she herself subverts this trope in that she is a rather average-leveled mage and swordswoman, and is a bit of a grump.
In X1999, Princess Hinoto has white hair and is one of the most powerful magical beings in that world.
Kisara of Yu-Gi-Oh!, the Mysterious Waif with connections to the Blue-Eyes White Dragon. Unfortunately, her white hair and pale skin led to discrimination and loathing by the people of ancient Egypt.
X-Men's Ororo "Storm" Munroe is a Dark-Skinned Blond(-ish) with, yes, blue eyes. This was eventually explained as a property she got from her mother's inherent magic. Her mother was an Egyptian princess.
Rogue from the X-Men has white streaks in the front of her hair. In the first X-Men film, this happens to Rogue after her power is drained by Magneto.
Songbird of the Thunderbolts. Though that's not her real hair color (just about all of the Thunderbolts had their faces altered to fit their scheme) and it has a red streak through it.
The X-Men villain Spiral, although she was originally a brunette before her genes were messed with.
Alpha Flight's Walter Langkowski, aka Sasquatch, once became a woman after returning from the dead (long story). Going by the name Wanda, she was young and pretty, and had snow-white hair in both her human and Sasquatch forms.
Ice (Tora Olafsdotter). White hair, blue eyes, and she's the princess of a magical northern tribe.
The first female Dove, Dawn Granger, in DC's Hawk And Dove. In her earlier appearances, only her Dove superhero form had white hair, while Dawn was a blonde. In more recent appearances pre-New 52, she was shown with white hair in both identities — most likely because it made it easier to differentiate her from the other two major blonde female characters in Birds of Prey when she joined that team. In her appearances in the New 52 reboot, colorists went back to only coloring her hair white as Dove.
Hippolyta (Lyta) Hall, the second Fury, who featured prominently in The Sandman. It's not entirely clear whether this is actually blonde hair drawn as white (since a younger Lyta is shown as a blonde, and one character refers to her as a blonde as an adult), or if it really is white.
Abby Holland (née Abigail Arcane), Swamp Thing's wife, has snow-white hair with two black streaks running through it.
When the Metal Men had to assume human identities (due to humanity turning against them and believing them to be dead), Platnium became a fashion model named Tina Platt. Fittingly, she's a platnium blonde.
The Faerie Queens and Ladies in The Dresden Files are described as beyond gorgeous and white-haired (although the Winter Lady streaks hers with dye.) And the hair of Thomas Raith's girlfriend, Justine, turned white after Thomas fed on her so deeply that she should have died. And a demon girl named Tessa has white hair in her rather cute human form.
The entire Marat race in Jim Butcher's other series, Codex Alera, tend to have white hair so a lot of them qualify as either this or White Hair, Black Heart. Kitai is the most prominent example, though there are several much less significant female Marat who are also this trope.
The protagonist of the wuxia novel The Bride with White Hair by Liang Yusheng. There is a Film of the Book too.
In Anne McCaffrey's The Rowan, the titular character has white hair, which is also inherited by her descendants of both sexes as a white streak.
Also in her Acorna Series, the title character has white hair and is a human-unicorn hybrid, with healing and other magical powers through her horn, though the whiteness of her hair stems for her people's tendency to go white after spending large amounts of time in space.
Daenerys Targaryen of A Song of Ice and Fire. Targaryens tend to have silver-blond hair as well as violet eyes due to their Valyrian blood, which has been maintained through the centuries with massive inbreeding. They were also The Beautiful Elite, and their rise and fall was closely associated with their control over dragons, which is ambiguously tied to the power of magic in the world.
Lady High Priestess Evelyn from White Star and the other novels in the series by Elizabeth Vaughan has white hair with a matching white dress. She says that she used to be a brunette, but one day, she pushed her Healing Hands to the limit to save a man who had been mortally wounded. She passed out, and woke up to find the man healed and her hair permanently snow white.
Miranda in L. Jagi Lamplighter's Prospero's Daughter trilogy. In the Back Story, her brother Eramus had turned it to an old woman's hair in anger; she had been able to restore its health, but not its color. When Eramus restores its original black, she doesn't like it, even though she recognizes the intent of generosity.
Kiki in the Kiki Strike series. It wasn't really much of a plot point, but it was used to show how unusual she was.
Jenna, from Jane Yolen's Great Alta Saga, has pure white hair and black eyes, a sign of her status as a messianic figure; she is a heroine of legend who is reincarnated every few hundred years, always with white hair from birth.
The Last Unicorn's Lady Amalthea has white hair, which makes sense, given that she's actually a unicorn that was turned into a human.
In Patricia A. McKillip's "Oak Hill", Elaine's dye, she claims, will find its color on Maris's hair. What it does is turn it white, like a spider web.
The Moonwoman in Deerskin, a goddess or nature spirit, has white hair. Lissar also gains white hair after the Moonwoman heals her and takes her memories away, which makes people think she is not human, or perhaps the Moonwoman herself. Only when this spell is broken and Lissar remembers her past does her hair return to being black.
Inverted in the first couple volumes of Kat Richardson's Greywalker series — Will, the man that main character Harper begins dating in the first book, is young and has striking white hair; but he's practically the most mundane, least magic-touched character in the series. And his inability to believe or cope with magic things comes between him and Harper. Turns out not to be such an inversion when Will is transformed into a powerful magical guardian. His white hair is the feature singled out in the final vision that Harper has of him after the transformation.
Chiana in Farscape. A feature of her species, apparently - the women have white hair and the men have black hair.
Yuki from the live-action version of Cutey Honey had white hair when she "flashed" into her battle form.
Daenerys Targaryen (natural brunette Emilia Clarke in a white wig) in Game of Thrones, who has silver-blonde hair due to her Targaryen ancestry.
In Classical Mythology Epimeliads were nymphs associated with apple trees and protectros of sheep and goats. They usually had hair white like apple blossoms or undyed wool, showing their special bond with the flock and fruit trees.
The Viera from Final Fantasy Tactics Advance (and, later, Final Fantasy XII and Final Fantasy Tactics A2), who regard it as a blessing from the viera spirits. Also, Ritz, subverted in that she hates it (because she gets teased about it constantly, and resents the misery it causes her and her mother), and therefore has it dyed red up to the epilogue.
Micaiah of Fire Emblem: Radiant Dawn. She's called the Silver-Haired Maiden, and yes, that's a plot point. Severalplotpointswith the last one playing a very important role in the plot.
For that matter, there's also Ninian the Dancer from Fire Emblem Elibe, and the shaman Diadora and her daughter Julia in Fire Emblem: Genealogy of the Holy War. Not quite white in these cases, but close enough. Similarly, Ishtar from the same game is either portrayed with whiteish-pink or silver hair. Also Idoun of Sealed Sword.
In Neverwinter Nights this trope is rather common among female Aasimar and other celestials in Faerun. The half-celestials Melissa from the Trials of the Academy module and Kaelyn the Dove from Neverwinter Nights 2 Mask of the Betrayer both also have white hair.
Touhou has several such characters. Eirin Yagokoro and Fujiwara no Mokou have white hair and are probably the result of being immortal, though that raises the question of why Kaguya Houraisan has black hair. Sakuya Izayoi, despite supposedly being a normal human, has silver hair as well, which has lead many fans to speculate that she may not, in fact, be human at all. Letty Whiterock is a yuki-onna. Youmu Konpaku is a silver haired half-ghost (we don't know how that works either), and a straight up albino if you go by the fighting games' art. Momiji Inubashiri is a white wolf tengu. Then there's all the characters with very pale but still coloured hair...
In Harvest Moon, there's The Witch from Animal Parade, who lives deep in Fugu Forest and has silver hair.
A New Beginning brings us Olivia. It seems to be suggested that silvery hair, combined with dark skin, is how most people look in the Harvest Moon version of the Middle East—the vaguely Middle-Eastern-seeming Amir and Sanjay, along with the Wizard from Animal Parade, all have white hair.
High Inquisitor Whitemane, one of the last bosses of the Scarlet Monastery dungeon in the World of Warcraft, has white hair.
Following Tides of War, Jaina Proudmoore has silver-white hair with one blonde streak.
The sequel adds Zenia, who kinda fits this trope in that her origin is completely shrouded in mystery.
The sequel's final boss, Parace L'sia, also has white hair while in her human form. It changes to pink when she turns into her near-naked-elf-in-a-bathing-towel form though.
Arcana Heart 3 adds Weiss, who is a liberated experimental subject of a project trying to grant ordinary girls the power to channel Celestial Energy. This installment also reveals Lieselotte and Zenia to be prototypes in the same project... and long-lost siblings.
Valkyria Chronicles has Selvaria Bles, with long white hair, a general at the ripe old age of 22. Alicia's hair goes white only when she's "powered up", oddly, but it wouldn't be much of a surprise otherwise. Edy Nelson also has white hair, but she's normal.
Aliasse from Valkyria Chronicles II. Silver hair and red eyes seems to be the mark of a Valkyria, either all the time or when a girl turns into one.
Riela from Valkyria Chronicles III is a strange case; her normal hair is red at the top and turns silver midway, but when she goes into Valkyria form, her hair become entirely silver.
Riven in League of Legends has white hair. Considering her story involves her turning her back on her home city after seeing them abandon their principles of Social Darwinism, she's probably closer to the wise-beyond-her-years variant than anything else.
Noire and Uni from Neptunia, at least when they activate HDD.
Lady Calista in The Last Story. White hair, and a powerful magic user—in fact, her magic is so powerful, it can temporarily make people invincible! But she's not the only one—many magic users in The Last Story have pale or silvery hair (from party member Yurick, who has silvery hair, to the random NPC magicians, who have shocking white hair), so it seems to be implied that magic frequently has that effect on hair.
The Vessels of Etrian Odyssey: Legends of the Titan usually have white hair. The only possible exceptions to this are potiential Player Character choices: like all classes, Arcanists have Palette Swaps to pick from. However, all NPC Vessels have white hair.
The Nasuverse seems to love this trope, and it often indicates that the character in question isn't human:
Ilyasviel von Einzbern from Fate/stay night, as well as her mother Irisviel. In their case it's genetic, since Einzbern homunculi are descendants (or, more appropriately, clones) of Lizleihi Justica von Einzbern, who had the same white hair and red eyes. They also carry parts of her personality and even memories.
Ping from Megatokyo had white hair early on, before her body started changing in response to her character development.
Lei'ella from Inverloch, which marks her as one of the Severed, mortal elves with silver hair and golden eyes.
Kili Stormcrow, a shaman of The Dragon Doctors, has bleach-white hair due to an incident as a child—Kili developed spiritual vision so strong that Kili's perception was trapped in the spirit world. Kili's hair has become permanently white.
Zenobia in Hazard's Wake has white hair, possibly related to her (most likely) being a demon.
Joyce is prematurely grey, even though, Alien Abduction aside, there's nothing especially magical about her, although she does develop into an All-Loving Hero over the course of It's Walky!
Homestuck: Rose and Roxy both have white hair like Dave and Dirk, and have shown an interest in magic, with Roxy being a big fan of wizards and Rose dual wielding magic wands as her Weapon of Choice.
Calliope except that it's a white wig, and she's actually bald. She wears it to cosplay as her Trollsona who bears a striking resembalance to Calmasis, a character in Post-Scratch Rose's books.
Cielle, the clairvoyant magic shop owner from Broken Saints, has white hair, courtesy of albinism.
Nasha of SPAZ has white Rapunzel Hair, and while she's not magical, she's definitely a little strange
Lilette Einmell from Tales Of Hetalia has white hair, signaling she is a demon. In fact, this series runs on this trope, considering all demons have white hair.
RWBY's Weiss Schnee has white hair to fit with her color motif, and among the four main girls, her fighting style involves the most Dust, their world's own flavor of magic.
Avatar: The Last Airbender: Princess Yue's hair color was a plot point. She was actually born (or rather still-born) with dark hair like everyone else in the water-tribe (and pretty much everyone in the four nations). When the moon-spirit saved her infant life, her hair turned pure white and remained that way all her life.
Charmcaster from Ben 10 is a silver haired sorceress.
Nefertina from Mummies Alive! had white hair and pale gray skin in her undead mummy form. There was some inconsistency in her coloring throughout the series. In some of the flashback scenes where she was still alive, sometimes, she was shown with black hair, and sometimes, she was shown with white hair. And yes, she, too, was voiced by Cree Summer (guess which other character Cree voiced that Nefertina resembled!).