The Boss's father was a member of the Philosophers' Wisemen's Committee. He was killed by the organization after explaining their traitorous nature to his daughter while she was still a young girl. He was the last of the original Wisemen's Committee, meaning he was a man of great wealth, power and wisdom. He was also Ocelot's maternal grandfather.
Behind the scenes
In Metal Gear Solid 3: Snake Eater, The Boss's father appears in a cutscene featuring the members of the Wisemen's Committee, during The Boss's speech to Naked Snake before their final battle. Each member of the organization gradually fades away as she describes their deaths, with The Boss's father (shown alongside a young Boss) fading away last, implying that he might have been the final original member to die.