Rex and Pugsy in Beasts Of Burden. Notable that Rex was a Miles Gloriosus in the first story, but has gotten better. All of the characters are regularly terrified by the supernatural occurences that surround them, but these two (especially Pugsy) are the ones most likely to be visibly afraid—and yet neither will back down when their friends or their town is in danger
Commander Blanx of the pre-Crisis Martian Manhunter comics is a villainous example along the same lines as Donald Pierce. A Dirty Coward and Combat Pragmatist who never fights fair—and indeed, doesn't fight at all if he can get someone else to do the dying for him—he's nevertheless very dangerous when cornered, nearly killing J'onn in one appearance and going toe-to-toe with Superman in another.
Empowered of the comic of the same name is a variation on this—instead of considering herself a coward while still being brave, she lacks in self-confidence in her super-heroing duties while still trying her darndest to be a superheroine—which, as her friends point out, generally ends up demonstrating that she's better at it than she thinks she is.
The New 52 version of Power Ring is an enormous wuss that seems fearful of everything. However, he wields a Ring of Power that runs on fear which makes him as powerful as the rest of the Crime Syndicate.
Kyle Rayner relative to other Green Lanterns. Kyle, unlike all Lanterns who came before him, could actually feel fear and doubted his abilities and heroism frequently. Eventually, his ability to feel fear made him the only lantern capable of spotting the greatest threat to the entire Green Lantern Corps, Parallax.
In The Superior Foes of Spider-Man, The Shocker is treated as a coward by his allies due to his unambitious nature and his preference for keeping his head low, even though he happens to be one of the most effective combatants of the team. In his words, he has no problem with others considering him a coward as long as he's able to live another day. Near the end however, events more or less drive him to cut loose, at which point he not only single-handedly defeats the entire team but was also able to defeat The Punisher with a single blast, though exactly how much of this was embellished by Boomerang (who in the end admits that he's making up half of the story) is true.