When Bart first stood up (by himself) to Nelson's friend which is a white/yellow weasel, 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.
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?"
The episodes he spoke in are in bold.