While far from my favorite fighter, I do like Bowser Jr. in Super Smash Bros. Ultimate. Though he isn't new to the series, he was in Smash 4.
I really like how his alternate costumes are just the other Koopa Kids. You have Ludwig Von Koopa, Wendy Koopa, Iggy Koopa, plus Roy Koopa.
And Morton Koopa, Larry Koopa, and Lemmy Koopa.
As a fighter, Bowser Jr. has some neat tricks. He mainly fights inside his flying machine. Which have some decent smash moves to them.
But he's also got some neat special moves. My favorite was the little mini koopa robots. They walk into they find your enemies. While they can be thrown against you, they usually end up helping more than they hurt.
When Kirby takes his power, he gets the clown cannon power. Which allows him to shoot a cannonball just like Bowser Jr. and the rest of the Koopa squad. It's powerful but a little weak to where it should be for both of them.
His Final Smash is Shadow Mario Paint. He takes the form of Shadow Mario, as he was in Super Mario Sunshine.
And a big painted x will be stage. Hurting the foes who touch it. But I found it lame and weak. One of the worst final smashes in the game.
With a power boost, Bowser Jr. could rise to the best fighters in the game, but he's still a decent fighter and I one I enjoy battling as.