Guan Yu was a general serving under the warlord Liu Bei uring the late Eastern Han Dynasty and Three Kingdoms era of China. He is known as a very confident man who was arrogant and invoking towards enemy but modest towards men who respected him.
His health got worse because he got hit by a poisoned arrow while defending a castle.
In his last battle, Guan Yu realised that Eastern Wu had secretly formed an alliance with Cao Wei and attacked Jingzhou while he was attacking Fancheng.Guan Yu attempted to break out of the encirclement together with his son Guan Ping and subordinate Zhao Lei but failed. They were captured in Zhang Town and executed by Eastern Wu forces after refusing to surrender.
(Almost all of the above is from Wikipedia so that means I do not have own these lines).
I thought his weapon looked very similar to WB, and I believe that perhaps Oda referenced this man to illustrate WB. After some research, his issue with health and his death with his son seemed interesting to me. PerhapsWB and Ace will die in this war, or maybe not. Correct me if this seems like BS and remove this thread.
You can look at how Guan Yu looked from here:
http://www.wxf.org/xq/articles/ar4/a4j8.jpg