Ryu and Ken, and the Street Fighter series in general, can be considered evolutions of characters and gameplay developed by Technos for the 1984 arcade game Karate Champ. Besides the side-perspective pioneered by Karate Champ, common features include protagonists in white and red gi, highly specific command inputs, bonus rounds, and in the Karate Champ sequel Player Vs. Player, a number of locations from around the world. It is also worth noting that Masutatsu Oyama's bull-fighting demonstration appears in Karate Champ as a bonus game and Mas Oyama, famed for his ascetic training regime, is one of the main, real-life, inspirations behind the character of Ryu. The roots of Street Fighter also lie in Irem's 1984 title, Kung-Fu Master, produced by 'Piston' Takashi Nishiyama. After leaving Irem, Takashi would take on the role of director for Street Fighter at Capcom. When interviewed for the SF25 retrospective artbook, Takashi said:
I was taking martial arts classes at the time, so I guess you could say I wanted to make games about a topic I already knew.
Chun-Li was originally designed with orange as her default colour rather than blue, evidenced by her profile pictures in the arcade version of World Warrior. In-game her qipao is blue as normal but on character select and in her win/lose splash screens she is clearly wearing orange. This would seem to indicate that the switch to blue was made very late in the game's development. Note also that the colour of the outfit that she discards in her World Warrior ending is yellow-orange. The anomalous profile pictures were recoloured for the port to SNES in 1992 but players were soon able to select the orange scheme in-game when it resurfaced a year later as an alternate colour in Super.
M. Bison's appearance is implausably similar to the character Yasunori Katō from Teito Monogatari. In early depictions the likeness extends even down to the star on his cap (soon replaced by the Shadaloo 'winged skull' symbol). Considering the dates involved, the live action film adaptation, Tokyo: The Last Megalopolis, (1988) is likely to be the version that inspired key elements of Bison's design. Other sources suggest that general Washizaki ('Eagle Cape') from the Riki-Oh manga provided the inspiration. What is certain is that all three characters share many features in common.
The yacht in the foreground of Ken's stage from Super Street Fighter II onwards is Christened 'Buppo', the name is inscribed on the pilothouse of the boat. Ken's stage was almost completely redrawn for Super so this detail is missing on the older yacht in earlier versions. The real Buppo was a character designer on Street Fighter II and a longtime Capcom staff member. His picture along with the rest of the staff can be seen on the credits of World Warrior, Champion Edition and Hyper Fighting (if you don't lose a round) and is inset on the picture shown here. Funnily enough, in the Merlin sticker album for Super the letter 'B' has inexplicably been airbrushed out, so it reads 'uppo' instead!
On a similar theme, Bill Cravens, whose name and image is grafittied on the Block Heads pub in Birdie's stage in Street Fighter, was vice-president of sales and marketing at Capcom USA and, as a distributor, was a key figure in reviving the arcade business following the crash in 1983. He is also said to have sold one of the very first Pong machines at the dawn of arcade gaming in the early '70s. Bill Cravens passed away in 2007 but a kind of immortality is assured by being one of only a very few 'real-life' individuals to appear in the Street Fighter universe, with Mikhail Gorbachev's cameo in Zangief's Street Fighter II endings being the most egregious!
The two characters fighting during the intro to Champion Edition and Hyper Fighting are commonly said to be Joe and Mike, two opponents from the original Street Fighter. If true, Joe and Mike would hold the distinction of being the only characters besides Ryu, Ken and Sagat to appear in both Street Fighter and Street Fighter II. Curiously, in the international Sega Genesis/Megadrive port Special Champion Edition, the 'Mike' character was re-coloured and given a different hairstyle to appear caucasian, presumably to avoid any possible complaint of racist undertones. In the same vein, when Final Fight was ported to SNES, international versions depicted the enemies Dug and Simons with noticeably lightened skin tones and G, a thug in Final Fight 3 received the same treatment, betraying a distorted conscience about matters of race and violence on the part of censors in the early to mid '90s.
A pallete glitch in CPS-1 Street Fighter II causes Ryu and Ken to sometimes throw red Hadokens (sometimes called 'Golden Hadokens'). It's extremely rare; the odds are certainly lower than the 1/100 that is sometimes cited. Red Hadokens behave identically to regular blue ones, doing no more damage, but are nevertheless a memorable quirk. They are commonly confused with Ryu's Shakunetsu Hadoken, a fire-based variation of the move added in Super and likely inspired by the glitch. Update! In late 2013 Akira Nishitani (Nin-Nin) began tweeting interesting titbits to do with the development of Street Fighter II. In one such tweet he revealed that the red Hadoken was not a glitch but was intentional whimsy on the part of the programmer.