I think I've mentioned Uiuo Mercutio a couple times.

So this guy originated in a shorty one shot I made, where everything could be loosely tied to the Avengers in theme, like all the characters and the place names were all around Marvel comic book characters, and I'd reveal it all at the end for a good groan.

Mercutio's first name is written down in all my notes, but I'm not gonna try to pronounce that. Mercutio is kinda like Mercury, which is also called quicksilver, which was the joke here.

But I didn't want it to be obvious, so he was only slightly faster than normal people. He was also only slightly stronger, and slightly more durable. But he could cast resilient sphere, regenerate, and etherealness at will, and feeblemind once per day.

And he spoke in a shitty high pitched german accent.

So the one shot I mentioned before took almost a year to actually get through because of the shenanigans the characters pulled, and they didn't really care about Mercutio. I brought him back in the next campaign I did, with different people, where the goal was taking the infinity stones back from the previous PCs, who were basically all powerful.

The characters leveled pretty quickly, so Mercutio got to the point where he couldn't actually hurt them except for once a day with feeblemind, but with the other three spells nobody could lay a hand on him meaningfully. This new group of player absolutely loved and hated this character, and absolutely hated and loved every encounter with him.

Eventually they found him on their boat, his feeblemind failed, they knocked him out in one hit (he fell off the boat), then the one with the power gem jumped overboard after him, caught up to him (since he was unconscious), and snapped his neck and tore his head off. Everybody cheered, which is how you know you're doing it right.

Later they found out that Mercutio was a mechanical host of an ethereal gnome. They found the gnomes and the workshop that produces Mercutio later, where they found a less whimsical Mercutio occupied by an ethereal gnome who was actually willing to get business done. They killed this character almost immediately, being level 20 characters with five infinity stones, but if they had encountered him first, they'd have been totally hosed.