Ivysaur is the best of the 3 pokemon and can do everything by itself, so that's a common one people go to. It can ko as well or better than charizard. Charizard is not as good but it's still about around as good as the other heavies. I'm pretty sure that's not what they were going for, but they didn't nerf ivysaur's killing power much at all, so it seems they're fine with it being stand-alone.
shrug
Can just guess, but I think both Pyra and Mythra will be fine standalone, though for Mythra it really depends on how she can ko, that will make the whole difference. She could even have combos into confirms that Pyra doesn't have. But if she can't ko well, then most will probably rely on pyra to get those.
And Pyra in that case would probably work fine as a standalone, like charizard, even if she wouldn't be a higher tier character. Being a good player makes up the difference
I'm not so sure about Ivysaur being the "best" these days, I've heard some competitive players starting to think Squirtle is actually the best.
Myself I think it's more of a case of being very matchup dependent. When facing ultrafast combo characters I've been getting myself to use Squirtle more (not just for initial damage racking) and I'm really seeing how low-commitment moves and tiny size really helps in not leaving yourself open.
Ivysaur is an easy comfort pick because, being the all-rounder, it usually has something it can use that it's opponent doesn't or can't answer to as well. But unless said opponent can't deal with projectiles at all, I have a hard time seeing it as the best choice most of the time. It works better as a mix-up, or to capitalize on openings made by Squirtle.
Charizard of the three is actually more comfortable dealing with swords thanks to his range and movement speed, and as with most heavies is really good at pressing the advantage. If things go well for him he can often take down opponents faster than the others could on their own. Of course there are those low-level players who just use Flare Blitz all day.
I wouldn't say it balances
Ivysaur has decent mobility, charizard is only faster for running. Its range is what makes it so good. It doesn't have a triple jump, but it's dangerous to be around ivy anywhere in the air XD and the tether is amazing for recovery. And it has a lot more going for it too.
I'm glad the presentation is soon.
Charizard is faster than the others in every category except air acceleration where Squirtle beats him. Even his walking is not far behind Ivysaur's running. If Ivysaur was on it's own it's recovery would be way worse because of it's bad airspeed giving others plenty of time to chase Ivy down and otherwise get in the way of it tethering the ledge. The only reason people don't do it more often is because they're probably considering that you can switch to Charizard when things look bad here.
Ivy's poor movement also hampers how much it can it do in advantage. Yes up air and Vine Whip can be mean follow ups and juggles, but unless the opponent's airspeed is just as low as Ivy's, they can fairly easily get away after the first few hits, if any.
You could argue that Ivysaur's lower overall mobility (when compared to Squirtle and Charizard) balances out the high knockback that some of its attacks offer.
Anyway, until Sakurai's presentation airs on 3/4, there really isn't much else for me to talk about in regards to Pyra & Mythra.
The knockback distribution on it's moves is rather lopsided, though. This means players that know Ivysaur well can more easily tell what it will probably try to do to secure kills and make it surprisingly hard to do so. The reason it is slow is probably more due to the fact that it's one projectile, that the others don't have, is very versatile in both starting combos up close and harassing from afar with it's piercing nature.
I feel like I will use Pyra more since out of the two, I like her character much more than her tsundere couterpart.
They're
both tsunderes. Mythra's type is the more infamous and recognizable ("tsun", prickly or easily angered by default, "dere" or sweet and caring in select circumstances), but Pyra happens to be the opposite, less common type. She is calm and considerate by default, but becomes surprisingly stubborn and sometimes even a bit cold when things get serious. In the end I preferred Mythra because those moments where she lets her real emotions show were great examples "show, but don't tell" where her expressions and mannerisms just make her come across as a lot more genuine, without expressly spelling anything out.
On the other hand, I generally prefer heavier hitters in fighting games, so that would probably mean I'd feel more comfortable playing Pyra most of the time. But I'm sure I'll find the best places to use both in due time. Maybe Pyra will grow on me as a character some more with how much more exuberant and cheerful she is here.