The son of Doctor John Watson. Watson is a man of action, often jumping headfirst into a situation no matter the danger. This would normally be a cause for concern, but his reputable physical strength helps to counterbalance any difficulties that might come his way. He's also quite experienced in playing the sweetener to Holmes's bitterness, and because they have a nigh-unbreakable bond of trust, that's just fine by him.
He takes a liking to the heroine from the day they first meet, and he's always more than happy to help her.