William was the Lord of Tiberias and titular Prince of Galilee from 1187 until his death at an unknown date. He supported Guy of Lusignan

Guy of Lusignan

Guy of Lusignan was a Poitevin knight, son of Hugh VIII of the prominent Lusignan dynasty. He was king of the crusader state of Jerusalem from 1186 to 1192 by right of marriage to Sibylla of Jerusalem, and of Cyprus from 1192 to 1194...

 over Conrad of Montferrat

Conrad of Montferrat

Conrad of Montferrat was a northern Italian nobleman, one of the major participants in the Third Crusade. He was the de facto King of Jerusalem, by marriage, from 24 November 1190, but officially elected only in 1192, days before his death...

 for the Kingdom of Jerusalem

Kingdom of Jerusalem

The Kingdom of Jerusalem was a Catholic kingdom established in the Levant in 1099 after the First Crusade. The kingdom lasted nearly two hundred years, from 1099 until 1291 when the last remaining possession, Acre, was destroyed by the Mamluks, but its history is divided into two distinct periods....

 and joined Guy in besieing Acre in April 1189.

He was the second son of Eschiva of Bures (1118–1187) and Walter of Saint-Omer (1110–1174), he was possible a favorite of his stepfather Raymond III of Tripoli

Raymond III of Tripoli

Raymond III of Tripoli was Count of Tripoli from 1152 to 1187 and Prince of Galilee and Tiberias in right of his wife Eschiva.-Early life:...

. He was one of the nobles to escape from the battle of Hattin, he died before 1204. His older brother Hugh II of Saint-Omer (also Hugh II of Tiberias) was te first in line to be Prince of Galilee (Tiberias).

Sources

• Riley-Smith, Jonathan
  Jonathan Riley-Smith
  Jonathan Simon Christopher Riley-Smith, K.St.J., Ph.D. MA, Litt.D., FRHistS is an historian of the Crusades, and a former Dixie Professor of Ecclesiastical History...
  
  . The Feudal Nobility and the Kingdom of Jerusalem, 1174–1277. Archon Books, 1973.