Not much is known about this buck. Just that he's part of Celestia's secret armor program. His designation is SubjectSigma, hence the name.
As a colt his parents dissapeared in the everfree forest. He protected his sister, Soloris, from threats he could handle. Anything else he could only help. Once old enough, he joined the gaurd. Still only a recruit, he was chosen to be a test subject in Celestia's secret armor program, Horns and Shields. Anything beyond that is unknown.
Relationships with other ponies
Soloris-His little sister who depended on him when they were foals. Now she hardly sees him.
Celestia-He is the most succesful test subject in the Horns and Shields program, next to SubjectDelta. Celestia usually makes him go through tough training but never gained a true friendship with him
Luna-Sigma was at one point one of Luna's friends and most trusted personal guards. His loyalty to Luna was the primary reason he was chosen for Horns and Shields.
SubjectDelta-SubjectSigma's best friend. They have trained alot and been on many missions together. They usually seperate themselves from the other subjects however.