So far in this campaign of my m8s, I've played a sea-monster hunter (on a land based campaign) who was real salt of the earth with a slight undercurrent of menacing (Moby Dick themed character), who was infamous for scrawling everywhere "Ichabod was here." Followed up by ultra-wholesome front-line banner carrier (to offset the moral ambiguity of the other party chars) / #1 cheerleader of the Earth god Nolyth: Cayetan Carota, the Fortunate Centaur, who stacked endless rerolls and nevertheless was retconned from existence by a beholder's disintegration ray. Really buggered that one up!
Now that the other party chars have moved from moral ambiguity towards moral goodness, I decided to roll up an actual villain character. One of the player characters is a human Paladin that is the reincarnation of an ancient human Paladin. It started off as a stupid joke idea, and after two years I realised with grim certainty that his character - Dr. Klaus, was genuinely a Santa Claus parody. A year ago I even said his character seemed like a demented Krampus and he had to bite his tongue on just how accurate that call was.
Anyways, Krampus here had a fun little gimmick where any time he saw an Elf there was a small chance he'd roll to attack them. This happened once, and never again, but we kept getting little snippets of deepest Klaus lore here and there over the two years. We were all convinced that Klaus was a little demon Krampus but eventually between myself, the player and the DM, we all had accrued a whole bunch of lore about Klaus which turned him from a joke character into a piece of a wider epic fantasy puzzle. Amusingly, all three of us don't know what the others know and won't spoil it out of character, which leads to a real prisoner's dilemma where between the three of us the full story exists but no one person has it. The gist I know is that Klaus fought an ancient war against the snow elves, which ended in the complete destruction of the snow elves owing to the general dickery of the elven king and the general ruthlessness of the human-dwarven alliance. Klaus also tried to save the elves, so it gets complicated.
Given that the party is in an isolated tundra town full of only undead and cultists, any new character I'd have to introduce would be from one of those two camps. Cultists were vetoed by DM, so it was undead! And the DM and I both discussed the idea of making an undead character old enough to know this Klaus in his previous life. Whilst working it out with the other players, I ended up taking what was a joke character and entered into a strange mood. Perhaps the effort of having to continuously add ć with tildes activated some Tolkein brain but my joke of a "Britney the Banshee" char ended up evolving into serious char
Brittineć here being pronounced the same as Brittinor. I had to consult with all of my players beforehand in order to introduce such a wonky character; the Paladin Klaus is going to have to carry Brittineć's coffin around if they are to ever leave the mansion (he agreed, guilty that he helped exterminate her entire race of snow elves in a past life). The half-elf player and I came up with an idea that would help him deflect in-character blame for getting the party's most beloved char killed and would allow my char to look beyond their disgust at half-elves. The wizard player Drack would likely horrify Brittineć as back in her time human wizards were rare and the magic system wizards are using with spell slots and words did not exist in her time, looking like vulgar magic - but these differences could no doubt be smoothed over if Brittineć showed the wizard where the mansion wine cellar was.
All in all, got everything set up for a fun character debut. Went all out on the theme too; everything revolves around hellish hounds, loud screeching, spookiness or madness. I think subconsciously I'm giving up on stealth and subtlety to fit in with the way the party usually does things (my first char was stealthy and climby, my second was loud and four hooved legs do not a climber make, this char fights by making loud screams heard in a 300ft radius). Really leaning on the banshee/revenant/ghostly huntsman theme to get that good aesthetic.
Using playable ethereal banshee was pretty weird balance wise, so I suggested to my DM that though she was ethereal, she can do what her cognitive dissonance will allow. E.g. weapons will pass through her, but she believes weapons will harm her which damages her hold on reality, to explain why she's taking damage from normal weapons despite being ethereal. She
could pass through walls or locked boxes, but doesn't, because she doesn't think she's a ghost. Also gave her a 95% carry capacity reduction just to hammer in the whole "spooky polterghost" vibe. So despite being a skilled archer, lockpicker or hunter, I don't think she's strong enough to carry most weapons or kits anymore unless she grabs a mage hand or something.
I'm also bloody terrible because I always advise my new players to keep backgrounds simple, but then you just get into a strange mood and accidentally write an epic as a backstory