When Bart first stood up (by himself) to Nelson, and Lisa hailed Bart in the cafeteria as "My brother, the bully-killer!", the Coward cheered Bart on, then escaped (along with the rest of the kids) when Nelson came looking for Bart.
He joined Bart's army and took part in the training, but the Guard Dog exercise (where the troops ran along a fence with a barking dog on the other side) got to his nerves and he chickened out.
Bart slapped the Coward to make him shape up, but Grampa slapped Bart and rebuked him, because sergeants aren't supposed to slap soldiers.
Grampa ordered Bart to apologize. The Coward accepted and everything was fine. The Coward took part in the attack on Nelson and the Weasels (1 and 2), and Bart's army won.
When Bart took a picture of Homer dancing with Princess Kashmir and showed it around school, the Coward was one of the kids who wanted a copy.
When Lisa showed off her new horse, Princess, to other kids, the Coward was among the onlookers and he said to Ralph Wiggum, "Wow, she really tamed that horse." Referring to Lisa, Ralph replied, "Yes, but who could tame her?"
Angry and about to fight.
In the gym.
Running in the hallway.
Hanging out with Chuck.
In Bart's army.
Coward in the mobile pool with the other kids.
Coward (on the left corner) giggling with his classmates. Also seen without his glasses.