From the Metal Gear Wiki

The Metal Gear Mk. II is a miniature Metal Gear designed by and controlled by Otacon. Its design is loosely based on Metal Gear REX. Otacon uses it to deliver ammunition and advice to Solid Snake. It is revealed at the end of the first trailer for Metal Gear Solid 4: Guns of the Patriots that the Mk. II's processor is the Cell processor, the CPU used in the PlayStation 3. Otacon explains that the MK. II's CPU is "the key to winning the console war" as he demonstrates with a small clip from the upcoming PS3 title, Killzone 2.

The Metal Gear Mk. II can also be used as a short range reconnaissance device, use Stealth camouflage, and shock enemies, knocking them unconscious as shown in the recent gameplay video. It runs on batteries which can run out of power while in operation, similar to the Motion Detector and Thermal Goggles, which, among other items, consume battery life in Metal Gear Solid 3: Snake Eater. Snake has been shown controlling Metal Gear Mk. II with a PS3 controller however, through a limited range.

[edit] Behind The Scenes

Metal Gear Mk. II also appeared in Hideo Kojima's Snatcher. in that game Metal Gear is the robotic "navigator" assigned to the protagonist of Snatcher, JUNKER operative Gillian Seed.

Metal Gears
Main Series
Shagohod — Metal Gear RAXA — Metal Gear TX-55 — Metal Gear D — Metal Gear REX Metal Gear RAY — Arsenal Gear — Metal Gear Gekko — Metal Gear Mk. II
Non-Canon Metal Gears
Metal Gear Chaioth Ha Qadesh — Metal Gear GANDER — Metal Gear KODOQUE