John Sheridan was the second commander of Babylon 5, a role he took from the first episode of Season 2, up through the end of Season 4. In the fifth season, he took on the role of president of the Interstellar Alliance. He was portrayed by Bruce Boxleitner, who unlike much of the Babylon 5 cast, was an established, big name actor. Discussing a fictional character, it is usually a good idea to keep the story internal and story external aspects of the character separated, something it is difficult to do with John Sheridan.
The first season of Babylon 5 was meant to try to establish the concept of a more cerebral science fiction television show, but the male lead, Jeffery Sinclair, and the actor that played him, Michael O'Hare, were both a bit too cerebral. He was a stoic character who acted more like a monk than a warrior. For both story related reasons, and for reasons of marketing the television show, it was decided that he should be replaced with someone who would more closely fit the action figure mold. And that was John Sheridan, as portrayed by Bruce Boxleitner. This was in fact lampshaded several times, where the character was referenced as a "jarhead". Unlike the former Jesuit-in-training Sinclair, Sheridan wasn't afraid to bust head and became more ruthless as the series progressed. Whether this counts as character development is something that is more debatable. The basic character development of Sheridan, in both arc and non-arc episodes, was that he gradually moved away from being a soldier towards being a leader and statesman. His most important relationship is in his relationship with Delenn, the Minbari ambassador, who he later marries.
He is also tied in with various religious symbolism as he returns from the dead. And overthrows The Shadows, the main "external" adversary, and also overthrows the fascist Earth Alliance president Morgan Clark. After which time he develops a galactic federation dedicated to promoting peace, all with the help of his beautiful, exotic wife.
And this is where I must inject my opinion in: John Sheridan is a bit what we would call a Mary Sue, a character that is beautiful, flawless, brave and intelligent, and always triumphs against his evil adversaries. And gets the girl. Babylon 5, on the whole, was meant to inject shades of value and ambiguous characters into the world of space opera, but John Sheridan, as a character, never seems to live up to the promise of the show. His growth from jarhead to diplomat doesn't seem as real as the development of other characters in the show. He does grow a beard, at least. Which is not to suggest that Bruce Boxleitner can't act, or that J. Michael Straczynski wrote the character badly. The character was not wooden or two-dimensional, it is just that as the lead character of the series, he is not the most interesting. That honor probably goes to the treacherous, romantic, humorous and doomed Londo Mollari.
As a side note: this being Babylon 5, there is probably a good chance that at least some of the exaggeration in his character as a square-jawed savior of the galaxy is meant to be somewhat ironic. Not total mockery, but there is some oblique criticism involved.
So the most important point about John Sheridan is that despite the twists and turns and depths that Babylon 5 brought to television, in and out of the science fiction genre, it seems not to have gone so far as to transform the male lead away from the in-charge-male archetype.