Alan Scott was a Green Lantern who arrived in the Anti-Matter Universe and who wore a red shirt and a cape.
While his real name was never explicit stated, nor were any details given about where this self-proclaimed Green Lantern came before arriving in the Anti-Monitor universe, he had apparently managed to do some good. His actions were enough that Gil Broom was inspired by him to become the eponymous Steam Lantern.
Due to Gil's excitement at the abilities that Hal's ring demonstrated, its to be assumed that his powers matched those of the Green Lantern Corp.
Hal did not recognize the physical description given by Gil, implying that he was not an official member of the Green Lantern Corp as far as he was aware. It is not completely clear if Alan came from Hal's universe, though it is assumed.
How Alan gained his Green Lantern power ring and powers is never explained.
- 116. "Steam Lantern" (mentioned)
Behind the scenes
Since the series was cancelled, he will never appear