This is taken from my Tumblr profile for Reno where I actively role-play as him. Headcanon.
A glimpse into the past
Reno was born to Jocelyn and Quinton Sinclair on a cold December morning in 1982. His father was a simple factory worker up until a tragic accident took his life when Reno was just eight years old. After that his mother struggled with bills and making ends meet, the little job she took waiting tables barely covered it all.
Reno was forced to grow up quickly, skipping school and playtime to help run errands for a little bit of extra gil. He also managed to get a little job at a garage every Saturday. His mother was against it from the start but as a devoted son he went against her wishes. Things became harder when his mother became ill and she had to stop working, Reno had to resort to theft and risked his life on the streets drug dealing. At age fourteen he soon became involved with a gang nicknamed 'Slum Mafia' and his crimes became more dangerous and large-scale. It paid for his mother's medical bills and put food on the table and that was all that mattered.
During his run with the Slum Mafia he gained his first tattoos, two crimson claw marks accentuating his cheekbones.
Unknown to Reno he caught the interest of Veld, leader of the Turks. Veld had witnessed Reno lead a group of gang members in a mini-raid of a rival gang's hideout and in the process the redhead stole a car. Veld asked a couple of other Turks to keep watch of Reno and report back. Reno's record was impressive; he even hijacked a small aircraft piloting it like a professional.
One day when Reno hadn't long turned seventeen, he was approached by Veld and offered a position within the Turks with promises of a healthy pay check, more than enough for Reno to get his rapidly deteriorating mother into a comfortable home. He had to give up his surname but it was an easy sacrifice to make.
Jocelyn Sinclair passed away peacefully and full of pride for her son three months later.
Reno went on to become one of ShinRa's finest Turks earning himself a promotion to his current position of second in command to Tseng.