So says Ridley Scott, but it was never the intention of the script writer for him to be so, and Harrinson Ford doesn't see him as one. Not only that, him being a Replicant totally ruins what this movie is about, a guy who has forgotten what its like to really live, to be human, who in the end comes to rediscover his humanity through people who are not human at all.
His character works because, in the end, they are both searching for the same thing, and only through them can he find it. That's what makes the movie so powerful. Otherwise its just a sci-fi action flick with a cliched plot twist.