Once upon a time...

... that is, in the 23rd century, mankind builds the very first giant warrior cyborg, called "EkHeart". Just one specimen (money won't grow out of apple seeds in the 23rd century either). The funny thing is that the cyborg has been - rightly - called like that before they knew why. His heart in fact will prove the main reason of his most adventures.

And the unsuspectable enemy is... (later with it)

As for the spelling, Eckhart was a German mystic in the Middle Ages, so the Army probably thought that it was a fitting name for a war AI machine.
But, who can tell? Nomen, omen.