Why they are called Ghosts ? Ghosts are rangers that have a biological phenomenon called Ghosting. It means having no fear at all, it is clear that the 'Ghost' term is used to deliver a sense of invisibility. Ever since an Ursa is blind and smells the fear,a ghosts are completly invisible to an Ursa.
The known Ghosts
The first 'Ghost' appeared on the surface of Nova prime, Cypher Raige it is told during the first starting narration of Katai, Cypher's son, who goes on to become a 'Ghost' himself later on Earth.
Both the rangers became 'Ghosts' during extreme conditions, Cypher became one when an Ursa was chasing him and shot a fear pincer into his shoulder and followed him off a cliff, after falling both Cypher and the Ursa fell into the river and the Ursa tried to drown him. During the events of After Earth, Cypher explained how he was astonished that this is how he is going to die and with a massive courage pulled the pincer out and gaind the 'Ghost Phenomenon.'
Katai himself on Earth found himself faced with an Ursa Maximus while he was trying to find a 'Beacon' the Ursa chased Katai into what looked like a volcano after Katai seemingly ditched the Ursa into the lava, he started to climb out of the volcano, only to be suprised by the Ursa who blasts out of the opening hurting and slamming Katai on the ground, seperating him from his C-40 cutlass and the beacon. The Ursa started to smell Katai but memories of his older sister who was killed by an Ursa rushed to his mind and he gained the 'Ghost Phenomenon' after that fighting the Ursa head on and killing it.