Tam Lin, Matt's mentor and guardian, is not someone you want as an enemy. He's huge and menacing and smart in an "I can think of all sorts of creative ways to make you disappear, permanently" sort of way.
But Tam Lin is also loyal and kind and really funny. What's the deal with this guy? Matt, too, isn't sure at first.
He spends the first few weeks tiptoeing around him. The man is just so large and dangerous looking, like having a tame grizzly bear in your house (7.23). Despite the fact that he's kind of scary and gruff, he's also funny and kind-hearted:
"Could you teach me?" asked Matt.
Tam Lin let out a bellow of genuine laughter. "I could if you wanted to learn how to break desks with karate chops. [...] I'll be around to hang you out the window by your ankles if you don't study." (8.82-3)
Clearly, he looks out for Matt, so it's no surprise that Tam Lin quickly becomes Matt's friend, teacher and father figure. Throughout Matt's childhood, we manage to almost forget about Tam Lin's real role at the Alacrán estate. It's easy to see Tam Lin as the rough-around-the-edges guy who is Matt's very loyal friend and protector. It's harder to see Tam Lin as a tough and mean bodyguard who is loyal to El Patrón.
The trouble with Tam Lin is that he's a walking contradiction. Tam Lin has all these great qualities that make us like him, and he often makes us laugh. But then we find out that he's a terrorist who killed children (albeit accidentally).
What makes Tam Lin such a fascinating character is his past. He is who he is because of what he's done. But what he's done is very, very bad. So now, he's vowed to be very, very good. He explains to Matt, "I'm talking about moral consequences. I've spent years benefiting from the horrors of Opium, and now I have the chance to put things right. I mustn't pass it up." (24.38)
Tam Lin's dark past makes him who he is now. In this sense, he's like a role model for Matt. He can't undo what he did, but he can make better choices to make up for his mistakes, and the same holds true for Matt.
In the end, Tam Lin knowingly and willingly goes to his death by drinking El Patrón's poisoned wine after the old man's death. Of course, we can't stand the fact that Tam Lin dies because we just love him to pieces. Matt, too, is devastated by his friend and mentor's death. But Tam Lin did what he felt he had to do and in the end he died a decent man, not a villain. He accomplished his goal and atoned for his sins. Or at least we think so. What about you?
Oh, and here's an interesting little tidbit: In the novel, we're told that Tam Lin is Scottish, so his name has got to be a shout-out to the hero of a famous Scottish ballad. Cool, huh?