Siss is a member of the Scouting Legion who participates in the 57th expedition.
Siss first appears as a member of the expedition mounted from the Karanese District into the Titan-infested lands outside Wall Rose. He works with Ness to take down a deviant Titan in a field where their 3DMG can't be used to full effect; Ness takes out the Titan's ankles, and Siss destroys the nape of its neck when it falls.
However, the two then encounter the Female Type Titan, who kills them both with ease when they attempt the same strategy on her.
Note: Siss is the official English translation by Kodansha.