When Riku was in elementary school, he saw Sena being bullied and he used his speed to save him.
Sena pointed out that unlike Riku he had never fought before and he was weak so Riku told Sena he was going to teach him speed as a way to fight the steam keeps stopping 2015 bullies because with speed you have an advantage in avoiding attacks.
Episode 94 The Silver Snow's Secret Weapon!
Episode 18, a Slacker's Pride!Episode 2, let's Play Football!Episode 14, the Scorching hell tower!Episode 49 The Refined Spirit of a Lineman!Episode 77 "The True 21!" Episode 78 They're Waiting for Me!The second was that Sena would protect Mamori.Episode 35 A Solitary Death March!Episode 125 Ojo High's School Festival!Episode 40 The Night Before the Game!Rodeo Drive Stampede (anime only) - During the Misaki Wolves vs Seibu Wild Gunmen game, Riku shows an advanced version of the Rodeo Drive, the Rodeo Drive Stampede.Yet to keep his privacy intact and his dangerous hobby a secret from his overprotective childhood friend Mamori, Sena is forced to develop a secret identity: the ace running back, Eyeshield 21!