Johnny Sasaki is a recurring figure in the Metal Gear Solid series. In Metal Gear Solid and Metal Gear Solid 2, his role is that of comic relief. In the original Metal Gear Solid, he serves as a bumbling guard that at one point has his uniform stolen by Meryl Silverburgh, and catches a cold as a result. Later in the game, he suffers from a case of diarrhea while he's supposed to be guarding the cell in which Solid Snake has been imprisoned.
In Metal Gear Solid 2, he appears as a soldier on the Big Shell, and when Snake uses the directional microphone, he can be heard on two occasions. The first comes after the player encounters Richard Ames. During a conversation between Solidus Snake and Revolver Ocelot, Snake can point the directional microphone off to the side, at which point he can hear Johnny sitting on the toilet and talking to himself about his time on Shadow Moses Island and Meryl. Johnny's second appearance comes while Snake is assisting Emma Emmerich in crossing the connecting bridge. When she steps behind the middle pillar, Snake can again use the directional microphone to listen in as Emma encounters and has a brief conversation with Johnny that concludes when Johnny has an attack of diarrhea and runs away.
In Metal Gear Solid 4, Johnny, also known to others as Akiba, plays a larger and more significant role. He is a member of Rat Patrol, Meryl's squad sent to observe and keep peace in various war zones. Though he proves to be an asset to the team, he continues to contract diarrhea and performs silly/dangerous actions throughout the game. At a key point, he saves Meryl's life. Upon removing his mask, it is revealed he looks a lot like Snake in his younger days. From then on, he becomes a love interest of Meryl's.
In Metal Gear Solid 3: Snake Eater , which takes place during the 1960s, a great deal of Johnny's origins are revealed. During a hidden cutscene, while Naked Snake is in prison, the soldier who is guarding Snakes cell is revealed to be one of Johnny's forefathers. He has a son named Johnny and later states that all of the first born sons in his family are named Johnny, including himself.
Johnny's forefather also suffered from a bad stomach, although in an amusing inversion, while Johnny suffers from recurring diarrhea, his ancestor was plagued by constipation.