K comes from the Greek word kilo which means a thousand. The Greeks would likewise show million as M, short for Mega. So if we stay consistent with the Greek abbreviations, then billion would be shown as a letter G (Giga). Think of your computer expressing bytes of memory as kilobyte, megabyte, or gigabyte.

K was brought into the Latin alphabet with the name ka /kaː/ to differentiate it from C, named ce (pronounced /keː/) and Q, named qu and pronounced /kuː/. In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the sounds /k/ and /ɡ/ (which were not differentiated in writing).

k, eleventh letter of the alphabet. It corresponds to the Semitic kaph and the Greek kappa (Κ). It has changed its shape less perhaps than any other letter in the history of the alphabet. The Semitic form may derive from an earlier sign representing a bent hand.