An influential character in the Metal Gear franchise, and Solid Snake's senior in the US Army's Special Forces Unit FOXHOUND, known by a host of names, including Frank Hunter, Null, Frank Jaeger, and the Cyborg Ninja. He reappeared time and time again upon the field of battle, driven by his obsession to do battle with Solid Snake.
Metal Gear Solid: Portable Ops
Gray Fox began his career as a child soldier in Africa during the 1960s. His cold-blooded actions against the enemy earned him the title "Frank Hunter" which evolved into "Frank Jaeger," as a nod to the broken German he spoke. In 1966, Jaeger met Big Boss, who took him under his wing, but the boy was soon abducted by the Philosophers as a test subject for their Perfect Soldier Project. In 1970, having lost all memory of his past, he took part in the San Hieronymo Peninsula Uprising as FOX Unit's "Perfect Solidier" codenamed Null. Through his fight against Big Boss, after which the mercenary again saved his life, Null regained some trace of humanity and memory of his past.
Metal Gear and Metal Gear 2
When Big Boss later formed FOXHOUND, Jaeger joined under the codename Gray Fox. During this time, he adopted a young girl in Rhodesia after he had killed her parents, and named her Naomi Hunter. He took her with him when he moved to America with Big Boss and continued to care for her until she attended college. Gray Fox was given a solo infiltration mission during the 1995 Outer Heaven uprising, but was captured in action. After being rescued by Solid Snake, he disappeared and was not heard from until 1999 during the Zanzibar Land Disturbance. Gray Fox offered Solid Snake support from the shadows via codec, leading Snake to him to challenge him. After Snake defeated the Gray Fox-piloted Metal Gear D, the two fought bare-fisted atop a minefield. Solid Snake defeated his old friend and left him for what he thought was dead.
Metal Gear Solid
Gray Fox's "remains" were recovered by the Patriots and he was brought back to life by Dr. Clark. Pumped full of drugs, he was used as an experimental gene therapy test subject as a part of the genome soldier research. In the end, Fox slaughtered Dr. Clark, destroyed the facility and began to hunt down Solid Snake, leading him to Shadow Moses Island.
Armed with stealth camouflage and a reinforced exoskeleton, Gray Fox plunged Shadow Moses Island into panic under the guise of the Cyborg Ninja. He cut off Revolver Ocelot's right hand, and gave Solid Snake warning through short bust Codec communications under the alias "Deepthroat." He waited for Solid Snake in Dr. Hal Emmerich (Otacon's) office where he challenged Solid Snake to a fight to the death, even though he claims that he is neither enemy nor friend. The battle is cut short when he seemingly goes insane and escapes. He later assisted Snake in taking down Metal Gear REX by destroying its radome, which was used for it automatic targeting functions.
The Liquid Snake-piloted mech then crushed Fox underfoot, ending his life. Before he dies, he tells Snake about Naomi Hunter and how he was the one who had killed her parents. He wanted Snake to tell her that what he had told him. At the end of the game, Snake decides to not tell Naomi about what her adopted brother had told him, instead telling her that Grey Fox told her to tell her that he will always love her.
One of Fox's last line's, "Fighting was the only thing I was good at, but at least I always fought for what I believed in," resonated with Snake, who would adopt the line as his motto during his time in Philanthropy after Metal Gear Solid 1 and throughout Metal Gear Solid 2.
The cyborg technology that began with Gray Fox would have an increasingly important role throughout the Metal Gear time-line, from Olga Gurlukovich and Raiden becoming cyborg-ninjas in Metal Gear Solid 2 and Metal Gear Solid 4 to cyborg technology going global in Metal Gear Rising: Revengeance, replacing the loss of the Sons of the Patriots system.