Header image source- I tried to find the original source but had no luck, it's probably in one of the official strategy guides but I can't find any online!
I got to Metal Gear as a series very, very late (I'm just about ready to play Metal Gear Solid 4, maybe sometime this year) and so I completely missed out on pretty much every iteration of Metal Gear Online, which pits players against each other in a variety of tactical stealth action game modes. The official servers for both the PS2 and PS3 iterations of the game are long gone, but fansites like SaveMGO have kept things alive.
One thing about the PS2 iteration (included with Metal Gear Solid 3: Subsistence) that's always interested me is a very unusual guest character. Reiko Hinomoto and her heel counterpart Rowdy Reiko from Konami's own women's wrestle-fest Rumble Roses- at that point a single PS2 game with its lone follow-up, Rumble Roses XX, coming out on Xbox 360 a few months afterwards- are unlockable characters in Metal Gear Online. According to GameFAQs, once you earn any Animal ranking in the online game, the next game you host will allow you to toggle these two characters on or off. Fittingly for a game with such a focus on CQC, both Reikos can use the signature Sunrise Suplex move on hostiles (although that's really just normal Reiko's special move). Other than that, they play just the same as everyone else, guns and grenades and all!
This 1Up interview with Hideo Kojima explains why both Reikos appear in the game- he had a conversation with Akari Uchida, producer of Rumble Roses, about collaborating with the Rumble Roses series but the MGS team didn't want to work on it, but when it came to adding a hidden character to MGO, Kojima figured this would be a good way to make good on that collaboration. Initially, he wanted to talk with Tomonobu Itagaki to see if he could get a Dead or Alive character, but went with Reiko instead.
... Y'know, for a series with only two games to its name and no signs of a return, Rumble Roses got more references in other games than you'd expect- Evil Rose is unlockable in New International Track & Field and a Reiko-inspired Bomber is in Super Bomberman R. Weird.