Species bond

A species bond existed between each Lupar and their designated human. The bond's purpose was to save them, and the Earth against an event called the Flux. Karvanista was bonded to Dan Lewis. (TV: The Halloween Apocalypse) However, humans were never informed about this due to being a Level 5 planet, so the plan equated to an alien abduction.

While they were species bonding, the Sontarans got behind the Battalion wall in the milisecond before it was made and launched temporal attacks across time. (TV: War of the Sontarans)

Behind the Scenes
The concept of species bond doesn't really work that well if given a thorough look; since there can't be an exact matching number of Lupari per humans. Does this include babies? Mothers pregnant with multiple children? What If someone recently died or there were not enough Lupari?