He first appeared in Pop Goes The Sneasel where he met Ash and his friends at the summit of the Ho-Oh shrine. He overheard the shrine guard ask Ash if he would help ward off what was preventing them from getting Ho-Oh's sacred flame. Immediately Harrison offered to help too. Later hearing that it was a Sneasel that was attacking those who approached the sacred flame, he had started to make plans on catching it. Once at the top of the stairs, Harrison instructed Ash to try to get the flame while he and Misty distracted Machop and Machoke. However he underestimated Sneasel's strength and he was forced to come up with another plan.
Using Houndoom and Phanpy, Harrison and Ash took on Machop and Machoke together; their goal being to take them both out before they would get to Sneasel. Both of their Pokémon were able to hurl Machop and Machoke down the stairs leaving Sneasel by itself. After Team Rocket's brief interruption, Harrison used his Blaziken to take down Sneasel and he was able to capture it. At the end of the day, Harrison told Ash that he would meet up with him again at the Silver Conference.
It was Harrison who convinced Ash to go to the Hoenn region. He told Ash about Professor Birch and all the Pokémon that Ash had probably never seen before. Harrison appears to have a strong personal connection with Professor Birch, whom he speaks of fondly, noting his words of wisdom was the main motivation for his travels to Johto onwards.
Harrison fulfills a supportive and mentoring role throughout his appearances, similar to Brock. He has also been shown to be quite independent, with no mention of any previous companions. In addition, he is quick-witted and confident, as seen in Tie One On, where he sends out his Miltank to stop Ash and Jackson from fighting. This quality is also seen in his battle style, as seen with his Blaziken's speed and prowess against Ash's Charizard when it recovers from a Seismic Toss by aiming a Flamethrower at the ground.
Despite his team of strong and eclectic Pokémon, he does not brag about his skills and capacity as a Trainer. Instead, he prefers to encourage others to do their best. His battle strategy is built on speed and reflex, such as Kecleon's camouflage and Sneasel.
Harrison has a streak of determination, as seen with his battle, and later victory, against Ash, as well as his eagerness to capture Sneasel. Harrison remains humble throughout his appearances, and is unfazed by his loss in the semifinals. He demands a lot from his Pokémon, although he gives them time to rest and recover, such as when he left his Blaziken out of the battle against Jon Dickson. Though this move ultimately contributed to him losing, he wasn't too upset about this. His compassion towards his Pokémon may be linked to his personal connection to Professor Birch, as explained in Johto Photo Finish, where Birch told him that in order to know Pokémon, one needed to see them through their own eyes, and hold them in their own hands.
Harrison's Kecleon first appeared in Can't Beat the Heat!. It was the first Pokémon Harrison sent out during his battle with Ash. It battled against Ash's Pikachu. Using its color-changing abilities and speed, Kecleon was able to hide almost perfectly, and fire some attacks at Pikachu from the tall grass, until Pikachu himself hid into the grass. Kecleon used its tongue to cut the grass, but Pikachu managed to grab Kecleon's long tongue and finish the Color Swap Pokémon with a Thunderbolt.
Harrison's Hypno was seen in Playing with Fire!, where Harrison sent it out to face off against Ash's Totodile. Totodile's Scary Face had no effect on the Psychic type, and after a stunning Hypnosis, Hypno was able to defeat the Big Jaw Pokémon with a Psychic attack. Ash next sent out Snorlax, which, however, was asleep. Harrison took the chance and had Hypno to use Dream Eater on the Sleeping Pokémon. This, however, angered Snorlax, causing it to wake up. It then defeated the Hypnosis Pokémon with a single Hyper Beam.
Harrison's Steelix was the fourth Pokémon Harrison used in his battle with Ash. It was sent out against Ash's Noctowl. After taking a hit from Noctowl's Confusion, Steelix used Sandstorm to blind the Owl Pokémon, and then hit it with Dig and Iron Tail in quick succession, knocking it out. Next Ash chose to use his Snorlax, which was quickly trapped by the Iron Snake Pokémon's Bind. However, Snorlax managed to escape Bind by using Ice Punch, and then it served the finishing blow on Steelix with a Hyper Beam.