It's always good to be a bit skeptical, no matter if the leak has (or does not have) your favorite newcomer.
There's a reason why I mentioned how it may not be the final roster, or that there isn't plans for DLC to be released shortly after launch. Sakurai is a man of surprises, and I feel like there is a real chance we don't quite have the big picture, assuming the leak is real in the first place.
I also feel like the actual character choice for the banner is really weird. Isaac, Banjo & Kazooie, and Geno are great... but Shadow feels a tad lazy, Mach Rider is out of nowhere, and im not sure how you'd even start with a moveset for Chorus Kids.
Also, I personally feel like characters like Shadow were insanely poor pick due to other series characters such as Tails being fan favorites for almost as long as Banjo Kazooie. Either he's a less-than-accurate echo or he's a unique newcomer almost stealing the stage for Tails. He was cool when I was, like, 12...but now he just seems like a continuation of the weird trend of echoes being the dark version of the main fighter, except Shadow is just straight up edgy without a reason unlike Dark Pit or Dark Samus.