As played by the incomparable Eddie Paskey, Lieutenant Leslie was one of TOS's most prominent background artistes, making his first appearance on the bridge in the second pilot ("Where No man has gone before"), and lasting all the way through to season three (IIRC, "And The Children Shall Lead"?).
Leslie could most frequently be seen in a particular background position during bridge scenes, often reacting with detached bemusement at whatever the main characters were up to in the foreground of the scene. His raised eyebrows and almost silent infered "Why me?" expression were a common sight in many classic episodes of TOS. And the few occasions he was allowed to speak were always memorable too, like the time he mouthed off at Captain Kirk in "This Side Of Paraside", leaving the good Captain rather shocked (nay, speechless
!) at Leslie's blatant insubordination.
But it's a little known fact that, as with Chuck Norris on the internet, Leslie is Star Trek's very own Mementic Badass.
Ex Astris Scientia has this to say about Mister Leslie:
Leslie occupies so many different positions as if he could run the ship all alone, as a navigator, a science officer, a medical officer and, of course, as a security officer.
To heck with him only running ONE ship alone... I think that Leslie could, if he really wanted to, run an entire Armada! Remotely, using only his carefully honed brain wave patterns!
how badass Lieutenant Leslie is.