What pokemon could fight using the weather?

i wanted the character to use a weather-based fighting style and it was originally going to be a castform but i changed it because i wanted a castform to be her mentor so what other pokemon would fight using the weather?

There are a lot of options here -and I mean a lot. A great many Pokémon can utilise weather effects one way or another - Swift Swim users in rain, for instance, like Kabutops, or Solar Powered Pokémon in strong sun, like Tropius. Abomasnow, Tyranitar, Hippowdon, Ninetales, Politoed - all of them have some direct connection to the weather as well.

You could broaden this a bit further by saying that most Grass-types can utilise the sun, if you think outside the box. Perhaps using Growth to increase leaf surface area could give a Grass-type sudden boosts in energy (which is, I admit, not totally realistic, but it's a nice thought), or something like that. The same goes for Fire-types, really, the sun drying out a battlefield and preparing it for a blaze. Or an Ice-type might freeze the rain to create icy spears. There are a great many possibilities there.

But - and it's a pretty big but - all of these Pokémon use only one kind of weather. Change it, and they lose their special something. Take your Sunflora out of the sunlight and put it in the hail and it no longer has much going for it (though you could argue for a way in which it might draw nourishment from the rain). If you want something like Castform - something that has a canonical ability to utilise all weather forms to support it in battle - there's a bit less choice.

You could start with Pokémon that can learn Weather Ball. These are: Lugia, Ho-oh, Castform, Roserade, Vanilluxe, Bellsprout, Snorunt, Celebi, Drifloon, Raikou and Bulbasaur. I'm pretty sure Lugia, Ho-oh and Celebi aren't quite what you're after, being somewhat legendary, so I'll discard them; Raikou and Bulbasaur only learn it by event, so you might want to discard them, too. That leaves you with at least five obvious candidates, excluding Castform, that all have a canonically-proven ability to use any kind of weather system to their advantage in some way. From that point, you're free to interpret that ability in any way you choose, as long as you can make the reader believe it's plausible.

That's not to say that's the only way to go about it, though. You could sift through the Pokémon that can learn the various weather-changing moves and see which of those can learn the greatest number of them, and which of them has any other defining characteristics that might make them suitable for what you want. Alternatively, if you were willing to confine yourself to one type of weather, then your options become a whole lot greater.

It really is mostly up to you. As long as you can convince the reader that there's a plausible reason for what you're doing, you can write whatever you like.

F.A.B.

For Rain teams:
Toxicroak is a good choice to use in Rain because, with the ability Dry Skin, he restores lost HP at the end of every turn. Combine that with Leftovers, and he recovers most damage. My favorite moveset for him is Swords Dance, Drain Punch, Ice Punch, and Sucker Punch. Once you set up one S-Dance, he pretty much OHKO's almost everything. Another viable option in Rain is electric types. You can use Thunder instead of Thunderbolt. For me, the two best electric pokemon in Rain are Rotom-W and Jolteon. Give Jolteon Timid nature and Choice Specs and she's a baws. Rotom-W has Hydro Pump and Thunder. Both benefit from Rain hugely. Also, pretty much in every team you need something that'll protect it, right? Well, in Rain, Ferrothorn is the best. He resists everything a Rain team is vulnerable to, and in Rain, fire type moves do less damage, so Ferrothorn always survives a Fire type attack.
For Sun teams:
Cherrim, Rotom-H, Venuaur, and Heatran are good in Sun. Cherrim's ability. Rotom-H just cuz, well, Rotoms are awesome! Venusaur because of his ability. And Heatran (holding an Air Balloon) resists Dragon types. And Dragon types are, like, my biggest problem in Sun. Cresselia also does very nicely in Sun. Pokemon that can learn Synthesis, Morning Sun, or Moonlight, I like to put in Sun. All the Grass type pokemon in Sun should have Solarbeam (in my opinion) and all the Fire type pokemon should have Fire Blast. An if they can, teach the Fire type pokemon Solarbeam also.
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This is just what I do for Rain and Sun. You don't have to agree with me though. And uh, hope I helped!

Sunflora, Castform, Dragonair (as seen in the manga) and any ice type (hail).

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