Harley debuted in the episode "A Cacturne For The Worse", in which he and May participate in the Izabe Island Pokémon Contest. He becomes offended when she doesn't recognize Cacturne and later calls it "scary-looking". He offers her a cookie and she says it's "not half bad" and he takes it as an insult. He takes a photo of her and puts it in what can be assumed to be a "burn book" and draws a skull-and-crossbones over her photo. (In some airings of the episode, he draws a sad face.) He vows revenge on her for insulting his cooking and thus begins their rivalry.
Harley is very scheming and will use any means to defeat and/or humiliate his opponent. In "A Cacturne for the Worse" he tells May that Surskit is slow so she should let her Bulbasaur get in close before striking, almost costing her the win. He also records her little brother Max telling an embarrassing story about her and a group of Tentacool and plays it during the contest, effectively stressing her out until Ash tells her to calm down. He also allies himself with Team Rocket in the episodes "New Plot, Odd Lot" and "Going for Choke". In "Hi Ho Silver Wind" and "Deceit and Assist", he tells May to use her Skitty's Assist move in the next match, causing it to become confused.
Harley has lost most of his contests against May despite his cheating and deception, except for one, in "Harley Rides Again". He still manages to get plenty of ribbons from other contests and has enough to enter the Grand Festival, as he has won over ten contests.
Harley's gestures, expressions, and speech patterns tend to match those of a girl. He runs in typical anime schoolgirl fashion and his hips tend to shake when he walks. His outfit is very flamboyant, with tight bell-bottom pants and go-go boots. He also has silly nicknames for May and her friends, calling May "Miss Tent" and "Gingerbread Cookie" (in the Japanese, he calls her "Meno-chan" and "Kamo-chan") and Ash, Brock, and Max "May's boys" or her "boy toys". At one point in "Harley Rides Again" he calls Brock "big boy" and joins a crowd of girls in admiring Brock's Bonsly. In season 9 he began adding "honey" or "hun" to the end of his sentences when speaking to someone. For example: (to Contest MC) "Well, don't look at me, honey." He also uses several cliches in his speech ("Off to go powder my nose. Toodle-oo!"). In "Begin! Pokémon Contest- Grand Festival", he cross-dresses as May to stress her out before the contest begins, blowing a few kisses to Drew just to throw in a little jealousy factor. His appearance, actions, speech, cross dressing, and his rather blatant fanboying of James dressed as Norman in "Hi Ho Silver Wind" lead many to believe he is homosexual. Whatever the case may be, flamboyant is definitely the word to describe him.
When May entered contests in Kanto, it was revealled that Harley was travelling through Kanto too. He tried to make her loose her chance to enter the first Pokemon contest they participated in but May got a Kanto Contest Pass and defeated Harley's Ariados using her Squirtle. The next time he appeared was in Harley Rides Again, where he competed in the Wisteria Town Contest, using Ariados in the appeals. He used Ariados's Spider Web to surround the stage with its web, similar to the previous contest. He then asked it to entangle itself with its web and use Scary Face, scaring everyone. He then used his Octillery against May's Munchlax in battle. After utilizing Octazooka, Sludge Bomb and Rest, he defeated May for the first and only time known so far.
He appeared again in New Plot, Odd Lot! and Going for Choke!. This time after learning that May needed one more ribbon to enter the Grand Festival, he devised a plan to prevent this. To do this, he enlisted the help of Team Rocket, promising to help Jessie win and having James and Meowth help him sabotage May behind the scenes. However Ash, Brock, Max and Drew got wind of this and thwarted his plan.
In New Plot, Odd Lot!, it was also mentioned that part of Harley's hate for May came from when a girl who looked like May ate his food when he was younger.
The next time was in May, We Harley Drew’d Ya!, Thinning the Hoard, and Channeling the Battle Zone, where he competed against May in the Kanto Grand Festival. In the Appeals Round, he used his Banette while dressed up as May, asking himself to be called Mayley. He used Cacturne and Wigglytuff against Munchlax and Eevee, but both defeated his Pokémon.
The final major appearance he had was in The Unbeatable Lightness of Seeing where he simply watched May and Drew have a practice contest battle. At the end of the episode, he revealed that he was also going to compete in Contests in Johto. He also invited Jessie to do the same, but she denied, saying she still wanted to stay in Team Rocket. Because of this, he got mad at her. He then sent out his Cacturne to battle her. As she was searching for a Pokémon to use, Harley asked Cacturne to use Bullet Seed, sending Team Rocket flying with Jessie still looking for a Pokémon.
He has Contest Passes to compete in Hoenn, Kanto, and presumably Johto as well.
In A Full Course Tag Battle!, he and Wigglytuff appeared in a flashback. Then Harley, Solidad and Drew made a cameo appearance in Pruning a Passel of Pals!, watching May perform in the Wallace Cup.
Harley tended to use nicknames for May behind her back. The first was Meno-chan (English equivalent: Miss Tent) after embarrassing her with the Tentacool story. The second was Kamo-chan, referring to the word often used at the end of May's sentences in Japanese that can imply unsureness. In English, he calls her various cutesy pet-names, including "gingerbread-cookie".
Harley seems to take immense pride in his Pokémon, so much so in fact that he dresses up in clothing modeled after his signature Pokémon, Cacturne. None of Harley's Pokémon share a particular type, but they all have one attribute in common: they can all be quite menacing in contests, even the normally cute Wigglytuff.
This listing is of Harley's known Pokémon:
Cacturne was Harley's main Pokemon. Harley seemed very fond of Cacturne, even dressing up as it. Cacturn proved to be skilled in contests and in battle.
Harley obtained Banette shortly before the Battle Frontier. Banette was a powerful Pokemon and skilled in Contest appeals, even taking its own Thunder attack and laughing instead of crying out in pain in order to impress the judges of the Hoenn Grand Festival.
Ariados first appeared in The Saffron Con, and Harley also used it in the following episode in the Saffron City contest. It battled against May's Squirtle, but the young Pokémon ended up defeating it by using Ice Beam. Ariados was sometimes used to subtly interfere with May's performances, using very thin strings to control May and her Pokemon on at least one occasion.
Wigglytuff's first and so far its only appearance was in Thinning the Hoard, where it was used in the Kanto Grand Festival. Unlike most Wigglytuff, Harley's Wigglytuff was notably more dark and aggressive.