The 'Pastel' part comes from my growing love of pastel colours and aesthetic over the past year or two. I don't use them in my artwork really, which is kinda funny to me, but I still love how they look!
I'm also just used to my online friends referring to me as Pastel by this point XD
The 'Plushie' part, naturally, comes from my love of plushes and soft toys. I just love to hug huge teddy bears and Disney toys XD
The '64' part is the most interesting to me though. You could think that it relates to my strong interest in video games, and while now I love how that idea has panned out for me, I actually just picked the number because I happened to like it when I was younger. I've used it for older online usernames like 'KerriTheCat64' and 'SophTheHedgie64', and never picked it for video game related reasons. If anything, I grew up on Nintendo GameCube/GBA/DS, so any consoles or Handhelds older than that I wouldn't have even known of back then XD