In AD&D II I had a female half-elven mage/thief. But she was a devoted follower of some elvish deity, and wore the clothing and even a genuine holy symbol of that clergy.

Imagine the surprize when enemies learned to wonder "since when can clerics throw fireballs?"

A slimoid with a fake beard

A human fighter called testi-cles. He was invincible to all but a nat 20, though a nat 20 was instant death for him.

When the dm actually rolled a nat 20 we had all forgot about a disadvantage till after he was dead, do it just got retconned to one of his testicles exploding and him surviving it

I had a little kobold cleric named Muk who thought if he was a good enough person he would evolve Pokemon style into a dragonborn

A tricky question. I've always treated my characters "seriously", in that I treat them as if they were real people with real histories and motivations and so forth, so even when they're seemingly silly in concept they are always ready to be serious.

I'm thinking probably the one who fits the "silly" moniker best was Puddle Dapplesky. She was a plasmoid paladin in a Spelljammer setting who was dead set on being an elf.

Not physically, obviously - she wasn't an idiot. Indeed, she was proud of her plasmoid physiology and often expressed bafflement at how attached everyone else in the party were to their bones. Bones only hold you back, she insisted. Other organs too, for that matter. She sang a couple of songs over the course of the campaign and one of my favourites was titled "You Don't Need a Brain to be Smart" in which she pointed out all the downsides of having a brain (you can't squirt it through small openings, one good blow to it knocks you out) and that real intelligence came from the heart (metaphorically of course since she didn't have one of those either). She'd often propose plans that required the party to get rid of their bones, sigh when they inevitably refused, and grudgingly propose an alternative that accounted for their inconvenient hinges and stuff.

No, she wanted to be an elf in the sense of being accepted into the Elven Navy as a proper Elven naval officer. She studied the rules and practised them fastidiously. The fact that she was crew on a pirate ship wasn't a problem because early in the campaign she managed to wrangle a letter of marque from the Elven Navy as a reward for helping them out on some issue that slips my mind. So she was a privateer. Entirely different from a pirate, and entirely by the book.

She had decided on this life's goal because a derelict Elven naval ship crashed on the asteroid her plasmoid colony had been living on since time immemorial (it was an extremely dull place, they didn't bother with a calendar since every day was exactl the same). There was a dead elven paladin on board and she claimed her plate mail, literally pouring herself into an Elven mould. The armor had some spiffy magical properties, including a "once a day when making a death save you get an automatic roll of 20". This would have been really handy if she didn't enjoy grappling so much, and as a result would very frequently in mid-combat jump right out of her armor (a bonus action due to her amorphous nature) so that she could get her plasmoid grapple bonus for grappling while naked. I had a separate icon representing her armor I would place on the battle map when that happened so I'd know where the armor was if she got the chance to return to it. Grappling wasn't usually the most optimal thing to do, but she liked showing off her plasmoid prowess. She was also shockingly strong, most opponents were surprised to be physically overpowered by a blob of blue jelly like that.

Later in the campaign she got a cool magic sword that was specially designed for plasmoids. It consisted of hundreds of razor-sharp flakes of steel that floated around in her body until she needed the sword, at which point she could "assemble" it like an extension of her body. Essentially a pseudopod blade, she could take it through one-inch gaps with her. It was a really cool sword. It was also cursed, the sword was possessed by a demon. She knew this when she bought it, it wasn't a secret, but she was quite confident that since she had bought the sword fair and square she possessed it. Legally. The demon kept trying to tempt her to do evil, and she kept reprimanding it and forcing it to fight for good instead. By the end of the campaign it was starting to crack, her resolve to follow the rules was proving stronger than the demon's will.

At the end of the campaign she ended up doing such awesome things for the Elven Navy that the Admirality - with a huge sigh knowing exactly what she would ask for - offered her a boon. She of course immediately requested a commission and was made a genuine captain of the Elven Navy, her life's ambition. They assigned her as captain of that one ship that every navy has where all the rejects and misfits that they can't drum out of the navy for whatever reason get relegated to. But she was happy for the challenge - she would be the elfiest elf on the ship and would whip all those other misfit elves into shape to show them how to be true elves as well.

A reanimated necromancer hand puppet who had to have a skeleton minion wear him in order to function properly

A skeleton wizard with the voice of Gilbert Gottfried.

Wadgour Raughters, halfling bard embarking on his solo career due to "creative differences" with his former bandmates from Blink Pig.

A guy fieri knock-off who was trying to go to all the major dining areas of every town to try them out.

The DM did not realize what I was doing until the game started and it was too late to change it. Strangely, the DM focused him in the first encounter until he died.

I played a halfling paladin named Cannip who was an initiate in a religion that preached that the way to reach enlightenment was to abandon worldly ties like armor and weapons and instead rely on your own muscles to get you through hardship. Their monastery was basically an outdoor gym with a wrestling ring in the center.

He was incredibly tenacious and very naive. I miss playing him.

One of my all time favorite characters was a monk based loosely on friar tuck.

He was chaotic good, sit around the camp sharing booze and smoked turkey legs and all sorts of other things (that he largely stole,)

And also would rob you blind if he saw someone who needed clothes or food or what ever.

And the bonking. Oh. The bonking.

Floridawoman Artificer, homebrew Firebug subclass. Before each session I looked up a bunch of headlines and added them to her character bio. Lots of fun to play actually

Not mine, a buddies. The character was a human beserker named "Vulcan Radiator" and when he would roll to go berserk he called it the "Overheater". That set the pace of how insane this campaign was...

Lanceafew! Cheesy paladin that took himself far too seriously.

Master oogway