Leo John was a young man living in British Columbia, and was the distanced nephew of Ray. Leo was known to have been obsessed with aquatic cryptids (particularly his family's myth of Sisiutl) and to believe that they were surviving prehistoric animals.
Leo entered the Predator Control Team's recruitment trials in the summer of 2011, but he was flunked from the trials by Dylan Weir for his unhealthy obsession with his belief that Ogopogo was a Basilosaurus.
Circa January 2012, when Leo got a video recording of a large sea creature which he believed to be Sisiutl in the coastal waters near his home, he went to Dylan about it in Vancouver to help him track down and identify the creature. Dylan was initially uninterested, until Leo showed her the raw recording of the creature.
Leo and Dylan subsequently drove back to the coastline where Leo had sighted the creature (in time to break up an argument between Ray and Harlow), and at a bay, they met up with Evan Cross and Mac Rendell. The group discovered an underwater Anomaly in the bay (which Leo believed to be the water moon from the Sisiutl legend) along with the marine creature that Leo believed to be Sisiutl. An amazed Leo immediately left to tell Ray about their discovery, but Ray did not believe Leo.
Leo later went back to the bay to try ad get more proof of the aquatic creature, and followed Evan into the local storm drains when Evan was tracking the creature there. After Evan and Leo met up and discussed the creature and the Sisiutl legend's possible accuracy, they tried to escape the drains without being discovered by the creature, but became trapped up a locked drain by it until Dylan freed them from the outside.
When the trio realised that the creature was a Titanoboa that hunted by sound and that it would follow the drain system to the Merison Oil loading facility, Leo immediately went to warn his family - who were protesting out the front of the facility - about the oncoming creature. Leo managed to get everyone at the protest to keep quiet and avoid drawing the Titanoboa's attention as the creature passed by in the sewers beneath them.
Later, after the Titanoboa was sent back through the Anomaly and the Anomaly had closed, Leo and the Special Projects Group realised that the Anomaly must have been recurring for centuries and been incorporated into the legend as the water-moon and the Titanoboa as Sisiutl respectively. As a result, Dylan instructed Leo to watch over the bay from then on and to call the SPG if the Anomaly reopened.
Leo was confident, eccentric and open about his personal views and beliefs. He was also shown to be gutsy and brave in the face of danger; initially taking being cornered up a drain by a Titanoboa as an opportunity to get more proof of its existence, and being afterwards thrilled and amazed from what he saw from the experience.