Stiener is a member of STAG who holds the "Sergeant" rank. The leader of the Saints go after him to assassinate him.
The Protaginiest hears that Stiener is making trouble for the crew, so he goes after him to assassinate the Sergeant. The Protaginiest travels to an Island, then calls Stiener to meet him at a location. The Saint leader managed to defeat Stiener in battle and kill him.
Sergeant Stiener is a very skilled man who is known to be an expert in STAG. He wanted to battle the Saint leader, and the STAG member who the Protaginiest called felt certain that Stiener would be capable of killing Sgt. Stiener.