Dr. David Keel

David Keel is a London doctor whose fiancée has just been murdered by drug traffickers. Grief tips into resolve: he wants the people responsible found and punished, and he's willing to step outside medicine and the law to do it. A chance meeting with the enigmatic John Steed gives him a partner and a way in.

Keel is the moral engine the whole series was built around before it discovered Steed was more interesting alone: a decent, angry, ordinary man improvising his way into danger. Ian Hendry plays him with plain, unglamorous conviction, all nerve and no swagger, which makes Steed's cool detachment land as a contrast rather than the default.