While visiting a town, Izzy stumbled upon an elder that asked him if he'd like a baby pokemon. Not ever having one, Izzy quickly agreed to keep it.
As an Eevee, Leafeon was usually seen being taken care of as it was only a baby. He usually watched his companions in amazement as they trained for an upcoming battle or contest. He has however been seen to be overly jealous when he isn't used for a battle and tries to show his abilities. He is also been seen at times to care for Izzy as he was his trainer and caretaker. Because of Leafeon being a baby Eevee at the time, he didn't really know any moves up until he battled against Izzy's Snorunt where it was revealed that he knew Tackle and Sand Attack.
While visiting in Sandgem Town, Izzy learned from Professor Rowan that Eevee was old enough to evolve. He told Izzy about Eevee's grass and ice evolutions and the locations of where he could evolve. Izzy then gave Eevee the decision of who it wanted to evolve into. Eevee placed his paw on Rowan's book with the image of Leafeon. Izzy left and headed out to Eterna Forest to find the Mossy Rock.
Eventually, Izzy returned to back to Eterna Forest and quickly found the rock. Eevee then evolved after touching it with his paw, becoming a Leafeon.