No companions have any points in persuasion (it affects success rate in Diplo, right?), so feel free to give it to any Int companion. I've heard people say nobles are better as emissaries; if this is correct, Sessai is your best bet.
There doesn't seem to be a good source on companions in Gekokujo except the wiki page with their stats, personalities and relations, so I thought I'd compile some information here. Since you can find their skill levels on the wiki, I'll just list suggested roles (descending order based on...
I tried to use this in warband, and the sounds were all messed up. I couldn't find a working warband version so I made one myself. If the author won't mind I'll upload the modified sounds.txt