Danny Pink worked as a Maths teacher at Coal Hill School where he met Clara Oswald and the pair became romantically involved. Previously, he had been a Sergeant in the army, serving both in the UK and Afghanistan, until he accidentally shot and killed a young boy while on a mission.
Fiercely protective of Clara, he managed to sneak on board the TARDIS using an invisibility watch. Initially, both Danny and the Doctor were suspicious of the other’s motives but after Danny helped the Doctor to defeat the alien war robot, Skovox Blitzer, a grudging respect began.
When Danny was tragically killed, hit by a car while crossing a road, Clara was heartbroken. When he was revived by the Cybermen and Missy as part of their plan to weaponise the dead, Danny’s will was strong enough to resist. He chose to sacrifice himself to protect Clara and relinquish his own chance of survival to allow the Afghan boy he killed to return to life.