Ammar ibn Yasir
According to Encyclopaedia Islamica, after the assassination of Uthman, Hasan played an important role in taking allegiance for his father, Ali ibn Abi Talib. One of Hassan's most important roles during the caliphate of Ali, was his representation by his father along with a group of four people, including Ammar ibn Yasir, to oust Abu Musa Ashaari from the Emirate of Kufa. During the Battle of Camel, Ali placed Hasan on the right wing and Husayn on the left wing of the corps. In the Battle of Camel, in response to the sermon of Abd Allah ibn al-Zubayr, Hassan delivered a sermon in defense of Ali ibn Abi Talib.