MRL
From Halopedia, the Halo wiki
The Initials on Master Chief's Boots are three letters ("MRL") hidden on the bottom tread of the Master Chief's boot on Halo: Combat Evolved. They are, indeed, initials - those of Marcus R. Lehto, Halo's Art Director (and the man behind the Master Chief's skin). An easy way to see the initials is if you're on a reflective surface (Example: on Two Betrayals, and on the Control Room's bridge) just look down and on Master Chief's boot are the initials.
