I have a lot of nostalgia for Bowser Jr, Sunshine was my first Mario game, and if he were to get into Smash, I always thought he should have a Shadow Mario transformation. Bowser Jr with a moveset based off his toys from NSMB, and Shadow Mario with the paintbrush.
When the game was announced, I honestly didn't care if Bowser Jr got in, Waluigi was my main choice for a Mario Rep. But then Rosalina happened, and Waluigi was deconfirmed. I thought Rosalina was going to be the last Mario Rep for at least this game.
I still thought Bowser Jr/Shadow Mario would be a cool idea, but transformations were deconfirrmed too. And I did not want Bowser Jr in the game, and changed my sig to show his slot as deconfirmed. I wanted him to be introduced in Smash 5, where he could have Shadow Mario.
Then Bowser Jr appears in the leaks, and he admittedly looks cool. I wanted to play as him. However, that all changed when the stream went up, and his Final smash was shown.
I can admit, I cried when I saw it, it was tears of nostalgia. When all hope was lost about my idea, I had accepted the Koopa Clown Kart, but then out of nowhere, my childhood comes for a surprise visit.
That is the reason why he is one of my mains, not only is he a unique and fun fighter, but he perfectly culminates what Bowser Jr as as a character. His entire history is shown, and he is definitely more deserving and unique than Rosalina is.