An Assassin [アサシン]

Ryo Hanaki was raised to be a professional assassin. One day he was defeated by some accident in the mission. Here he meets Rio, an attractive high school girl. He spent days as a heartless assassin, but his life starting changes after he helps Rio. By a twist of fate, Rio who was a complete unknown became an assassin target overnight. A single mistake from his decision to save Rio decided the destiny of the masses...