Boxer is very strong, but he works hard and always wants to know how he can help.
Boxer asks nothing for himself, even though he is one of the strongest of the animals. He always wants to work, and always wants to work harder and do more even without being asked.
A white stripe down his nose gave him a somewhat stupid appearance, and in fact he was not of first-rate intelligence, but he was universally respected for his steadiness of character and tremendous powers of work. (ch 1)
Boxer is modest because he never brags, and because he is Napoleon’s cheerleader. He encourages the other animals to work as well, because he is always working. The animals love and respect him, and Napoleon uses that.
Boxer represents the everyman. He is the typical hard-working citizen, and the backbone of the revolution. He's the first to start working and the last to stop, and does twice as much work as anyone else. He keeps things running with his hard work, and helps make sure no one ever questions the leadership. People respect him, so they respect their leaders.