In the grand scheme of The Shawshank Redemption's plot, Tommy (played by Ally McBeal's Gil Bellows) isn't a major character in terms of how long he's on screen. Some of the other cons have significantly more while being no more than narrative furniture.
The reason Tommy matters is because he's the key to Andy Dufresne's redemption and a walking, talking insight into his humanity. When Andy takes him under his wing to help him pass his GED exam, he forms an almost paternal bond with the younger man, and seeks to break the cycle of crime that would ruin him and his family.
And just as we're coming to love Tommy for his resolution to help clear Andy's name, he makes the mistake of telling Warden Norton of his intention to testify to Andy's innocence and he's brutally gunned down by twisted guard Byron Hadley. In a moment you see Andy's redemption evaporate (or so we think), and an affable young man's life wiped out in one.