I want to hear the golden trio Pokémon head canons
//Okay, so let's get Frisk and Chara out the way.
1. Frisk's Pokémon are entirely based on what you catch and what you choose.
The reason I say this is because Frisk is a blank slate protagonist, meaning anything that revolves around them is up to the interpretation of the player. Meaning also, Frisk will use any Pokémon, which is the perfect segway into Chara.
2. Since Chara and Frisk have the same soul, Chara's team will most likely mirror Frisk's. Why? Because they could also be up to interpretation. Despite the smile and backstory, they're still a blank slate in terms of personality and why they did what they did. You don't know that, and coupled with Frisk's actions, dependant on what you choose, depends on her team.
Say you do pacifist, Chara will have an entirely different team.
Say you do Genocide, and Chara will mimick your team and use it against you.
I assume Chara would certainly have a Dragon and a Fire type most likely due to the connection of the Dreemurr's and how some people view them as either Lions or Dragons.
3. Asriel would mostly likely have two Grass Types. And for me, the best fitting two would be -
Skiddo and Sunflora. The reason for this is because, Asriel is also based on a goat, most likely. And a certain friend of mine is very fond of them. And Sunflora to mirror the fact that Asriel went through a bit of a phase as Flowey. But unlike Flowey, Sunflora is cheerful and happy in the sunlight, so obviously a better choice for a companion. He'd also have a Dragon and Fire type, and I already explained why. Most likely give him Dratini and Litleo though. Here's how they look.
Oh and.. Asriel wouldn't like battling as much as his two siblings, but would do it to protect them, while Chara and Frisk both do it for fun, and sometimes he joins in.