If ur looking for a secondary for Gren, GnW or Mario is prob ur best choice. Fox is good too.
The MUs I personally want to cover are Snake and Mega Man (they’re not awful, I’m not actually convinced Gren loses either MU, but not worth the effort when so many better counterpicks exist) and maybe the thunder rats.
My problem is that I’ve never actually picked a character for “practical” counterpicking purposes in any fighting game I’ve played, I’ve always just played who I like. My best secondary is Wolf since I enjoy him and I played him in Brawl but he doesn’t cover the MUs I want to cover particularly well, although I find him very good against Mega Man.
How does Mario fare against Snake and Mega Man? I know he does well against the thunder rats and I do enjoy playing him. Fox I’m also considering, he fits my playstyle really well, I think his Snake MU could be more convincing though, I see it in a similar way to the Ness MU where it’s tough for Snake but not actually losing once they learn it.
G&W does cover the MUs I want to cover but I really don’t enjoy playing him so he’s out for me.
Ness is kinda fun but as mentioned I don’t think he covers Snake all that well and he seems bad against Mega Man. I’m considering Lucas though.
Mii Gunner has actually been working really well for me, he decimates Snake and does fairly well against Mega Man and the rats. My only problem is that Miis are somewhat impractical to have as secondaries rather than mains in bracket. I’d either have to set him up at the start of every set (and potentially have to set up multiple movesets since all 3 down Bs are good) despite not planning to use him much, or ask everyone whether they use Snake/MM before the match until I got to know the players. Miis can sometimes be subject to weird custom rules as well. These are logistical issues though which I could probably work around.
I find Young Link really fun who apparently beats Snake and Pichu, I can see him doing well against Pika and MM too so he might be the answer. I also just think he’s crazy good/underrated in general.