How to unlock Heroes in XCOM: Enemy Unknown

Using certain soldier names in XCOM: Enemy Unknown will activate XCOM Heroes – pre-designed super soldiers
such as Sid Meier and Ken Levine. Before activating a Hero, a pop-up will warn you that they’re too damn amazing
to allow you to continue the game with achievements turned on, and you cannot modify a Hero character after summoning it.
If you decide to use one, do avoid killing him with some bull-headed tactical screw-up —
we’d kind of like BioShock Infinite to be finished.

The currently known XCOM Heroes are below – if there are more, someone will surely find them soon.
Sid Meier
Ken Levine
Joe Kelly
Otto Zander