But Buck was neither house-dog nor kennel-dog. The whole realm was his. He plunged into the swimming tank or went hunting with the Judge's sons; he escorted Mollie and Alice, the Judge's daughters, on long twilight or early morning rambles; on wintry nights he lay at the Judge's feet before the roaring library fire; he carried the Judge's grandsons on his back, or rolled them in the grass, and guarded their footsteps through wild adventures down to the fountain in the stable yard, and even beyond, where the paddocks were, and the berry patches. What is Buck’s role in this excerpt?

Buck is the protagonist because he is the main character driving the action.
Buck is the protagonist because he is well behaved and kind to people.
Buck is the antagonist because he refuses to follow the judge’s rules.
Buck is the antagonist because he leads children astray on wild adventures.