McDonell Benedict Miller, or Master Miller, was first introduced in Metal Gear 2: Solid Snake as a drill instructor and survival coach for FOXHOUND and later introduced himself as one of Snake's radio contacts early in Metal Gear Solid.
While it was initially perceived to players that Master Miller starred in Metal Gear Solid as apart of Solid Snake's radio backup, it is eventually revealed during the story that it was Liquid Snake masquerading as Miller to manipulate Snake. The real Miller had been murdered, presumably by Liquid and his men, two days before the Shadows Moses Incident.
Despite never meeting face to face and communicating purely by codec, Liquid still saw fit to disguise himself as Miller by wearing shades and fixing his hair into a mullet. This was because players could still see a portrait of Liquid whenever Snake called him up, adding to Metal Gear Solid's quirky sense of self-awareness. It's also notable that while masquerading as Miller, Liquid's codec advice is largely useless to player, serving as little more than pep talks with no gameplay advice or benefits.
Peace Walker bio:
Miller plays a vital role in the management of the MSF and supports Snake (Big Boss) during his missions. The child of an American GHQ officer and a Japanese woman, he was born during Japan's occupation by US military forces and was given the name "Kazuhira" - a name that represents "Peace" in Japanese. Upon graduating from college in the US, he joined the Japanese Self Defense Force. After being discharged, he drifted from one region to another as a mercenary, eventually meeting Snake in Colombia.
Unlike in the original, Miller was voiced by Robin Atkin Downes in Peace Walker.
Miller can be assigned to any unit on Mother Base. His unique "Mother Base Deputy Commander" skill boosts the morale of all staff members on the team he's assigned to. Similarly to Extra Ops 67: Date with Paz, Snake can go on a Date with Kaz, the goal of which is to seduce him by using Co-Ops Communication to pay him compliments or using CQC to interrogate him, wearing the Naked uniform will boost his affinity. If the player knocks out Miller, which he somewhat enjoys, he drops the design schematic for the Patriot machine gun. S-ranking Extra Ops 68: Date with Kaz unlocks Miller's uniform which can be used by any male soldier on missions, including Snake. If the player wears the swimsuit during the mission, Miller will be wearing a speedo.