I consider Galmar to be one of the more entertaining characters in the game. He may come off as a frothing-at-the-mouth nutjob at times, but at least he's not boring. Along with the fact that Ulfric is hands down the most well-develeped character in the game (IMO), that pretty much guarantees that I'll always play the Stormcloak side if I do the Civil War quests. Their Imperial counterparts are just one long giant yawn in comparison.
He's an alright guy, but why Ulfric keeps him on his war council I'll never know. He doesn't show much tactical aptitude. He's more a rabble-rouser and disciplinarian.
He's Ulfric's housecarl, and was housecarl to Ulfric's father before that; apparently the two of them also served in the Great War together, and Galmar may well have been responsible for whatever combat training Ulfric got as a young man. Whatever faults he may have, I think his unquestionable loyalty to their family and the fact that they can probably finish each other's sentences by now probably goes a long way.