One particular character that's never really been utilized well in the anime is Giovanni.
:( This makes me sad, but that's probably because I like Ted Lewis as a voice actor, he was just never properly able to shine on his own for years for whatever reason--I think bad scripts are to blame, personally, but New York dubbing companies were really weird back in the day. He oddly enough sounds like he tried hard to imitate the Japanese voice in some aspects while still having that New York accent, then he voiced Tracey and so sometimes you can hear that in his voice.
Anyhoo, I
try to have a consistent voice for characters, both those who have a canon voice or don't, but I fail so badly at it lol. Think it's only worked fully with Digimon, but then that was until I heard the
Tri dub, and now Kari sounds weird in my head and makes me sad. (Not helped that she has
four different voice actresses that I'm aware of for the character, two Japanese, and two English. She's a mess in my head.)
Funnily enough, Mewtwo alternates back-and-forth between Philip Bartlett and Dan Green for me, even though I prefer the former's performance, and yet for some reason sometimes I get a blending of the two and don't mind it at all. It's weird, I don't know how to explain it, but I just get a "different" Mewtwo in fan fiction because of that (which is stupid to say because this is fan fiction we're talking about and even the best writers have a hard time getting his character down right, but it's just a weird feeling).
For the characters who
don't have voices, that's a bit of a hard one when it comes to Pokémon fics, since a majority of them have voices, so I turn to the Pokémon themselves. I don't have any specific voices in mind, but every time I read Lugia's lines from
anywhere, I don't think of the 2000 Lugia's voice, think of Kyle Hebert, and his voice goes all over the place, too. It's kinda funny to me.
In my own writing, Cresselia is Monica Rial. I have no idea why, but that's the face that comes to mind, doesn't matter
what I'm writing
which isn't much 'cause of Forsaken, but I've tried a few unrelated one-shots. She's versatile, too, so she's not
always having a scratchy voice.