Short answers, pulled from the story.
Ryu was created by Capcom game designer Takashi Nishiyama, who modeled the character's spirit on the South Korean-born martial artist Mas Oyama. Ryu's own name comes from an alternate reading of Nishiyama's given name, Takashi.
Ryu debuted in the original Street Fighter arcade game in 1987 as the lead playable character. He appeared alongside his best friend and rival, Ken Masters.
Ryu develops Evil Ryu because he is unable to fully control his dark nature, called the Satsui no Hado, the same power wielded by the villain Akuma. Evil Ryu was introduced in a 1996 Street Fighter Zero manga by Masahiko Nakahira before becoming playable in the international versions of Street Fighter Alpha 2.
Kage is a separate entity introduced as downloadable content in Street Fighter V, formed from the evil energy Ryu purges from himself. He represents the Satsui no Hado that Ryu rejected, and he fades from existence after failing to overpower Ryu.
Byron Mann played Ryu in the 1994 film, opposite Ken's actor Damian Chapa. Capcom's preferred choice, Kenya Sawada, appeared instead as a separate character, Captain Sawada, because his English was not strong enough for the role.
Yoshiki Okamoto said Ryu ended up weaker than Ken in Street Fighter II due to a development mistake, even though the two shared the same moves. Capcom had considered giving their shared Shoryukens a deliberate weak point, then decided against it, but the flaw landed on Ryu anyway.