But to sum it up: Rex was the captain of the 501st during the Clone Wars. Or in other words, he's a regular clone capable of keeping up with Anakin "Walking Disaster" Skywalker. Later he had his control chip removed and eventually joined a rebel cell.
Re: Gen, Finn meets Rex--OP note
http://starwars.wikia.com/wiki/CT-7567/Legends
But to sum it up: Rex was the captain of the 501st during the Clone Wars. Or in other words, he's a regular clone capable of keeping up with Anakin "Walking Disaster" Skywalker. Later he had his control chip removed and eventually joined a rebel cell.
Rex is awesome.