Stone

Overview
Stone is, strictly speaking, not a member of the Mangione crime family. Rather, he is a mercenary, a specialist, a gun-for-hire, and a master observation and cunning. There's rarely a job he won't take, and even fewer he won't complete with calculated, ruthless efficiency.
Background
Stone joined the military at a young age, and quickly rose through the ranks. His tactical efficiency, ability to plan ahead, and execute complex operations with the precision of a clockmaker. However, his ruthlessness and his willingness to do whatever it takes to complete a mission made him a feared among his peers. His reputation for getting the job done, no matter the cost, made him an asset in intelligence gathering, but made others unwilling to work with him. Fearing that you might end up being one of his 'pawns' he was willing to sacrifice.
After leaving the military, Stone found his way into both the criminal underworld and more 'gray' communities. Taking on jobs that others feared to touch, and completing them with little-to-no friction, made him a feared and respected figure in the criminal underworld.
Relationship to other characters
Stone and Johnny have a complicated relationship. Johnny and Stone served in the military together, being a perfect compliment to one another. Stone's ruthlessness contrasted Johnny's pragmatism, but that made the pair the perfect balance to one another. Stone's level of intelligence and planning is matched only by Johnny's. Their loyalty to one-another stems less from a sense of honor, and more in a transactional sense. From them saving each other's lives multiple times over their years of service together.