Senji Masamune is Sahara Sanjūrō brother.
Described as "geniuses", together the pair discovered a new form of energy derived from the earth called "magic stone fuel."
Utilising this source of energy, they began created other inventions such as airships, and eventually they began manufacturing things like fully automated armor, which would become the Automated Warriors of the current time.
However, countries began to argue with each other in the fight to obtain the pair of brilliant minds. Eventually, one side was able to obtain the technology of the automated warriors, and the fight escalated into a Great War.
While how they were able to obtain this information is unknown, it is heavily implied that it is somehow related to Senji, given that it would later be Saharah who would appear later to defeat the armoured warriors and oppose the enemy nation.
His current status is unknown.