Since his early conception, Street Fighter's Ryu has always been a hero at heart, similar to a typical anime protagonist. Always soft-spoken but tough, his style in the original game reflected this as it resembled a full-contact Karate stance. But by Street Fighter II, he broke the mold of tradition by mirroring the fighting stance of Bruce Lee from Enter the Dragon. While his outward appearance was the opposite of strong and silent, his core values never faltered and later created a weakness that put him on a path towards his greatest strength.
Since childhood, Ryu was raised learning martial arts along with his best friend and rival, Ken. Their master, Gouken, was an accomplished martial artist who instilled discipline and tradition in Ryu. That discipline has since set him out on a journey to consistently improve himself by taking on the strongest fighters around the world. But by doing so, he inadvertently fed a much darker side of his personality.
The desire for improvement often goes hand in hand with the desire for more power. Ryu's master saw this firsthand when his brother, Akuma, embraced his Satusui no Hado and let the dark energies corrupt his body in a bid for more power. Ryu has often danced this line with his own battles and occasionally dipped into the Dark Hado.
His embrace of the Satsui no Hado eventually gave birth to Evil Ryu. When this darker form was awakened, Ryu's urge for power and the intent to kill and obtain it became insatiable. Even after he purged himself of the darkness, he still struggled with the repercussions of such power and with keeping it in check. It would ultimately become his greatest weakness.
But his struggles through the darkness were not made alone as his closest friends and rivals helped him remember the type of person he really is. At his core, Ryu is one of the nicest characters in Street Fighter, and his strong sense of justice has often aligned him with other heroes in the series like Chun-Li and Commander Guile. Even though he is far more reserved, he will not hesitate to jump at the chance to aid his friends, and it's that mindset that has helped keep the darkness at bay.
Alone, Ryu endured would have turned him into the next Akuma in no time. It is a weak point for the character who is otherwise a hero. However, through that weakness, he understood that his greatest strength isn't in power or martial arts; it's his friends and rivals. With Ken at his side and Sagat always pushing him to improve, Ryu has been able to defeat the darkness within and prove that he is stronger than his weakest parts.