Kisa was a dolphin Gi befriended when she was a young girl. Gi was forced to leave after her parents had their research grants revoked and the family had to move.
In the episode A Mine is a Terrible Thing to Waste Part II, Gi says she saved her allowance for a year in order to go back to the coastline where she met Kisa. However, when Gi arrived, the newly-built smelter on the coast had polluted the water to such an extent, Kisa was dying.
It was Kisa's plight that led Gi to swear to protect the seas and their creatures.
Gi is first introduced in the episode A Hero for Earth as she is playing with a dolphin. The dolphin also gives Gi her Planeteer ring. However, it is unlikely that the dolphin was Kisa, as the timeline suggests that Kisa should have died from the pollution Gi found her in when she was still a young child.