Tock is a watchdog. Literally. He has a huge clock in his belly.
We could just quit there, and he's already one of the most unusual characters in young adult literature, but we'll keep going. He's also a watchdog in the way we normally think of it: he's a guard. But he's a guard against – yep – wasted time. Seem complicated and strange? That's because it is. And such is the city of Dictionopolis where words take on some extremely literal meanings.
When we first meet Tock, he's policing the Doldrums, weeding out time-wasters. But did you notice he has no problem leaving that area behind to go adventuring with Milo? Why might that be? Well, it's pretty clear as he relates his sad family history. He doesn't really have a home. Well, hey, all the better to join the traveling gang then!
When we hear a character's family history this early on in the story, we kind of expect that this guy will be a very well-developed character. But in the end, he's not so much. We learn more about Tock in this part of the book than we do for the rest of it combined. He's a great companion, but we don't get much more than that.
Tock is one of the first characters who teaches us a little moral lesson. He does this by describing to Milo how people think time works in the Lands Beyond: "once there was no time at all, and people found it very inconvenient. They never knew whether they were eating lunch or dinner, and they were always missing trains. So time was invented to help them keep track of the day and get places where they should. […] [W]e were given the job of seeing that no one wasted time again" (3.11).
What do we get out of this little nugget of information? Well, for one thing, we get a little reminder not to waste time. Hey, after all, there are people (or dogs!) whose job it is to protect it! But maybe more importantly, we learn that we can't think of things from only the angle we're familiar with. To us, time is time. But in the Lands Beyond, "once there was no time at all." We can't even imagine this, yet it's true. What are we supposed to do with that? Definitely gets you thinking.
Sure, Tock has a few flaws. Even though he's a native of the Lands Beyond, he appears to know as little about some parts of them as the tourist Milo does. He's also not the brightest dog in the world. His alarm doesn't automatically go off when he and his friends meet the Terrible Trivium, a notorious time-waster. It's not until Milo starts to figure out what the Trivium is doing that Tock's alarm starts to go.
So, he's a little slow. But we don't care. Because, in the end, he remains a good friend to Milo (for better for jail) and helps our protagonist get where needs to be (physically and emotionally).