Daft Donald is a mute bodyguard who works for El Patrón. He's also a good friend to Tam Lin. Like Tam Lin, Daft Donald is a former criminal from the United Kingdom. Because he's mute, people tend to ignore him or even write him off as dumb or crazy. His nickname "Daft" had to come from somewhere, after all. Even Matt doesn't think too much of Daft Donald for most of the novel. But this guy turns out to be a hero, and a very smart dude to boot.
Not only does Daft Donald help Tam Lin get Matt out of Opium, he is also one of the few survivors left at the Alacrán estate after El Patrón goes all cult leader on everyone and gives them poisoned wine. We hear about this mass murder from Daft Donald himself, who turns out to be a very good and intelligent storyteller. Tam Lin saved Daft Donald's life, he tells us, by warning him not to drink the wine. Tam Lin then effectively passes the torch to Daft Donald, who becomes Matt's new ally and friend.