In My Life in Dog Years, Dirk can be said to be tough, loyal, and extremely protective.
After Dirk attacks and chases off a thug who takes a swing at Paulsen, the author realizes he has a bodyguard. To see Dirk in action is truly one of the greatest moments in the young man's life.
According to Paulsen, Dirk is as close to a live nuclear weapon as you can get. But the dog is not exactly what you'd call a bosom buddy; he's certainly not over-friendly. He still growls at Paulsen, telling him in no uncertain terms to keep his distance.
Paulsen admits that he only touched Dirk once. That was when he patted him on the head while he wasn't looking. For this little show of affection, he received a growl and a curled lip for his trouble.
Even so, as we can see from the episode with the thug, Dirk is nothing if not loyal, as well as extremely protective. He follows Paulsen to school and waits for him before accompanying him to the bowling alley, where he again waits for him. Dirk may not be the friendliest dog you'll ever see, but his loyalty and toughness make him a worthwhile companion to have.