I haven't thought about this yet. So everything I write will be winged. All on the top of my head.

He was the 1st person to join their crew. Muerte is a tailor.
Whilst Wasson and his wife were sailing, they noticed how tattered their clothes had become and stopped by at the multiple islands. But one island they stepped foot onto was very quiet and ominous.. the plants were all dead, the houses all abandoned and the sky above cast no light. The clouds dark. You couldn't tell what time of day it was.
Wasson's wife told him to get back onto the ship because this island was giving her the creeps but Wasson was so curious about this land that he just had to look around. And who knows, maybe he could coincidentally find the fruit he's been looking for, or even find more mushrooms that are yet to be researched.
Wassons wife followed him, arms wrapped around his as they trampled around the muddy ground. When ahead noticed a bright red shape. The contrast was crazy, all these dull murky shades when all of a sudden this bright red colour appears ahead of you.
Wasson rushed to check it out, and when he got close realised there was a well kept house that didn't look abandoned at all. Maybe life? The house was surrounded by an ocean of Red Spider Lilies, they grew in the garden and decorated the house.
Wasson approached the house and gave it a knock. There was silence. Until he heard shuffling and an unusual mumbling noise.
The door opens and Wasson is greeted to a long lanky guy, covered in head to toe in black latex gear, white long hair.. mouth forced open by a metal retainer. The man looked surprised like he had seen a ghost

Wasson's wife is terrified, but Wasson himself can't contain his excitement. They all stare at each other.. Until Wasson breaks the silence and compliments the array of flowers just outside, which causes the intimidating man to blush and smile with his eyes. He allows them both in, Wasson gladly excepting his invite but his wife extremely hesitant.

They introduce themselves, Muerte communicating via a notebook.
Wasson asks about the island and Muerte tells him its been like this longer than he's stayed here. He explains that when he first got to the island, he noticed how it was just a ghost town which was being overtaken by nature. He used his devil fruit powers to check for any souls that still wonder the island and found quite a few. And after speaking to them, they told him everything. Muerte would then stay in the island, talking to all the lonely souls that weren't ready yet to move on. And he grew attached to them.

But also explains how he's been here for so long he wants to move on and do something better with his life. He's still very young and he isn't doing much by living on a desolate island, speaking to souls. He wants to continue on making clothes and fixing them.

And Wassons like holy shit. We need a tailor to help us out. Do you think we could pay you to do that? Or even better, would you like to join us on our journey.

Blah blah blah. Im gonna skip a whole load of stuff.

Muerte speaks to all the souls he met in the island and even offers to set them free. Because he doubts anyone else with a devil fruit like his is just gonna come and do the same thing he did. And if anyone were to come to the island it'd either be to destroy it or change it up. And he's sure the souls would rather not have to see that happen.

So Muerte sets all the souls free. The house and flowers he collects stay there too. And joins Wasson's ship as his tailor, and after the murder of Wasson's wife, second in command.