Flavius Julius Constans was born in 320 and was the youngest son of Constantine I.
In 337 he was a joint ruler of the western empire with his older brother Constantine II but when his brother tried to take advantage of his age and assumed military inexperience a battle between them ensued. It was Constantine however who was defeated and killed and Constans became sole ruler of the west.
In 350 while his remaining brother was busy fighting in the east, military general Magnentius rose up and declared himself emperor and unfortunately had the support of both the army and in not to short a time, the entire western empire.
The battle was over quickly as Constans lacked the support he needed and fled the region but was cornered in Gaul and killed.