That was a mistake - I implore you, play an Imperial Agent. Of course it's a matter of taste, but the Imperial Agent storyline is considered to be by far the best, not only by myself, but many players as votes on the topic indicate.
Technically you can of course unlock the class by other means. I have exactly three level 50 characters - Twi'lek Consular (Infiltration Shadow), Chiss Imperial Agent (Marksmanship Sniper), Chiss Jedi Knight (Combat Sentinel).
Oh, I've started one, the leveling is extremely slow, as I've been busy with other classes. It had nothing to do with the storyline however, and more to with "This is Star Wars! I need a lightsaber! I need force powers!"
I'm finding I don't care much for the duck and cover mechanic, but suffer through it with my Gunslinger. So, going to try Lethality Operative on my Agent.
The funny thing, after all this, so far my favorite class playstyle wise (though I quite enjoyed the storyline myself) is the Bounty Hunter.