There’s a powerful argument to be made that Ace, companion to the Seventh Doctor and posh girl pretending to be street-scruff (her real name is Dorothy, y’know), is the Scrappy Doo of the Whoniverse: she’s a terrier, likes a fight, has a big mouth and a lot of what I believe you Americans like to call “addy-chewd.”
She’s also pretending to be something she isn’t (tough, in Scrappy’s case, streetwise in Ace’s) and possibly only exists because the producers of the TV show were worried about losing the younger viewers.
There again, she got rid of the over-sibilant panto bickerfest that existed between the Doctor and Mel, and that’s no small thing. Plus she walks around with a bunch of explosives on her back, and you can’t imagine Nyssa ever doing that.
read the rest (complete with clips) at Anglophenia