Captain Martin Crieff

So John Finnemore himself has come out and said that the characters don't necessarily look like the people providing their voices. The only clues we have to their appearance are what's said in the show, and the clearest description we've ever got of Martin is that he's a "short, red-faced little man." However, I'm still using Benedict Cumberbatch as a PB, with these headcanony thoughts in mind:

Martin does look similar to Cumberbatch: in his face, and in the fact that he's proportionally skinny to his height. If you saw him standing next to one of Cumberbatch's other characters, you might think they were related. But there's no way he'd ever really be mistaken for, say, Sherlock on first sight. Because he's 5'7". Also his hair, as in his icons, is a medium reddish-brown.