So, it was kind of a tough choice, but all in all, my primary main is Mewtwo. I love Pokemon ever since I was a kid. And back in Smash 4, I was excited to see Mewtwo return as the first DLC character, as he wasn't in Smash Bros since Melee.
When I finally got him, I loved his playstyle and moves the moment I picked him up. This was when I was a casual, by the way. That made me easy to go back to maining him in Ultimate. He has his glaring weaknesses, but I still really enjoy playing as him.
I also secondary Inkling, mainly because I love Splatoon. Another one of my favorite series once I picked up the second game on the Switch. An easy character to learn, but I enjoy playing as them, and to help out with some of Mewtwo's bad matchups, and vice versa.
I also pocket Joker, but I rather stick with my two mains, especially online.