User blog comment:Chiaki Akira/So, in Riku's designation as "Spieler"/@comment-34612254-20200122190050

It essentially means "Player" in german. Riku and even Sora are seen as the players meant to bring about something. Riku brought the end of the war, and Sora and Shiro will lead to the end of Tet's Disboard. To Ex-Machina its like greeting a commander or general to ask for orders. This is also belived as to why he grew a crush on Riku and Sora