N'Kari is either a Keeper of Secrets or a Daemon Prince, depending on if you're reading newer or older fluff. She/He (either works, but since Keepers of Secrets are hermaphroditic, it doesn't matter) is notable for appearing in the canon before his/her backstories even happened, as well as swapping her/his Warhammer Fantasy and Warhammer 40k backstories around between the two universes and thus is the primary evidence of a multiverse connected to the Warp after the map of the Realm of Chaos.
N'Kari's name is taken from the villain "Nakari", an evil and seductive queen with sorcerous powers from the Soloman Kane novel "The Moon of Skulls" by Robert Howard (yes, the same guy who wrote Conan the Barbarian).
The Legend (Warhammer Fantasy)
In the Old World, N'kari was never mortal and instead a powerful Keeper of Secrets from primordial times that fought Aenarion the Defender a couple of times, then was struck down by him in the last battle and trapped in the fabric of a magical vortex created by Caledor the Dragontamer and friends to block Chaos from the World. She eventually got out, and to her props even Malekith was on edge about the whole thing. She proceeded to wreak unspeakable carnage in Ulthuan, hunted Anarion's descendants as revenge, and tried to destroy the Shrine of Asuryan, but that last one didn't work out too well.
As a result, N'Kari has played Saturday morning cartoon villain to the T&T twins (Tyrion and Teclis) ever since, appearing and taunting them, getting an asswhooping (and a corpse-humping from Tyrion most likely), then popping back into the Warp while shrieking about how he will be back and they won't be so lucky next time until the next time actually comes which ends up as just a rinse/repeat of the last time. Since Malekith was Aenarion's son, N'Kari tried to go after him once, but Malekith kicked her ass (even without dragon help) and she never went after him again.
He also made an appearance in Warhammer Online standing in a fountain staring at the sun, but whatever.
According to latest studies, N'Kari once was a human native of Old Earth, an actress of dubious morals going by the name of Sasha Grey. Such was the power of lustful pleasure associated with her that, eventually, the Warp reflection of her soul transformed into a sentient being of unimaginable passion and lust. After the realization of her own existence, she instantly sensed the perturbations in the Warp, which was the formation of Slaanesh, and immediately recognized it as her future master. N'Kari then patiently waited for the birth of the new Chaos God and totally and undoubtedly gave herself to him.
She did some funky stuff after that, and also was at Horus' Battle Barge at the final moments of the Siege of Terra, where she was banished by no other than the All-Mighty Corpse Emperor, and was sent back to Slaanesh to have a rape party afterwards. She also fought the Phoenix Lord Asurmen during The Fall, killing him for the first time because his backup, Drastanta, Phoenix Lord of the Shining Spears, was out getting smokes. Fortunately, Drastanta showed up in the nick of time and slew N'kari with his mighty lance before going all emo and go sulk forever.
She also was doing some scavenging on the derelict Craftworld of Malan'tai, where she was "Om Nom'ing" the Eldar Spirit Stones, until some of those retarded SUPER KNIGHTS OF THE EMPEROR swooped in, curbstomped her demonic servants, and then got their asses kicked by N'Kari until something random happened, and she was defeated.
There isn't that much fluff behind her, but
we can all assume she's generally badass, or not that badass, like any other Keeper of Secrets, but as they are Greater Daemons of Slaanesh, they're badass, but not as badass as Greater Daemons of other Chaos Gods. any truly interested person surely can find some really cool videos of her human days (though one wonders what would happen if she saw this page?).
It is rumored that the Fifty Shades trilogy were some of her daydreams kept in
(she/he/shim?) its diary before N'kari's ascension to daemonhood, and that foul heretic E. L. James (who is secretly an ardent devotee of Slaanesh) found them and decided to spread them in preparation for the corruption of humanity by She-Who-Thirsts. The Inquisition has a standing order that all copies of these obscene texts (and anyone who thinks they are actually novels and not the steaming piles of foul heresy they are) must be shot into the heart of the nearest star straight away. Unfortunately, N'kari has grown even more powerful as there is now a trilogy of Fifty Shades of Grey movies. Appallingly, her literary musings are neither well-written nor interesting nor even particularly deviant. The sales figures are conversely monumental, which is only further overwhelming evidence that the tomes are suffused with Slaaneshi magics.
She turns up in the new Horus Heresy book Slaves to Darkness as the consort/plaything for the newly-living-in-the-EyeofTerror Fulgrim. She assumes a fat slob parody of Fulgrim (think snake tail, 4 arms, Fulgrim's face and hair but fat). After Lorgar binds Fulgrim with his true name, N'kari gives him one of her black fangs as a means of semi-permanently binding her.
While he hasn't physically made an appearance in the game yet, he has been referenced numerous times in the Eye of the Vortex campaign. He was name dropped in the epilogue of the Lizardmen campaign and revealed to have manipulated Arkhan the Black into conducting a ritual to free him. This was all likely done to hype him up for Warhammer 3. While he hasn't officially been announced yet, the fact that he has been so heavily foreshadowed and that Khorne got Skarbrand for his legendary Lord makes it safe to assume he will come as the leader of the Slaanesh faction.