Crystal first met Monlee when she was training to be a capture pro. It is there where an Arcanine was rampaging out of control due to an eye injury. Due to this, Crystal fell and broke her arms. Monlee and Bonee decided to intervene and attack the Arcanine to drive him away. After Crystal woke up they led her to the Arcanine, which she managed to calm down. Crystal added him to her team after the incident and Monlee became her arms until she recovered.
Monlee first showed up in Slugging It Out with Slugma alongside Crystal's other Pokémon to help prove her capturing skills to Professor Oak. Monlee was used in the next round to battle a Chikorita that wanted to join Crystal's team. Chikorita used Razor Leaf and became shocked when Monlee punched the leaves away. Monlee then used Mega Punch and broke the ground up. Chikorita stayed put while the ground give way and when it stopped, Crystal smiled and told Chikorita that no Pokémon had ever taken Monlee's Mega Punch in the face before. Crystal then added Chikorita to her team.
In Suddenly Suicune I, Monlee battled against Suicune to try and help Crystal capture it. Crystal had Monlee attack Suicune as it was falling to the ground with a Mega Punch. Although Suicune was prepared to counter the attack Monlee struck the ground instead, breaking up the earth and throwing Suicune off guard. Crystal then attempted to capture Suicune, only to fail.
In You Need to Chill Out, Regice, Monlee was loaned to Emerald for his Battle Pyramid challenge against Brandon and his Registeel. Monlee began the battle by circling around his opponent and striking with his powerful and speedy punches. Emerald states that with Monlee's power the tide of the battle will be turned and has Monlee use Sky Uppercut. The attack sends Registeel several feet into the air and knocks it out. Although he won the battle, Monlee unexpectedly falls to the ground and turns sickly, due to Registeel's Toxic attack. With no items to heal Monlee, he falls to the ground defeated.
In Skirting Around Surskit II, Monlee worked alongside Bonee by Emerald to help him capture Jirachi. Emerald has Monlee use Mach Punch to prevent Jirachi from moving by threatening to attack with his super fast punches. While Monlee keep Jirachi distracted, Bonee whipped out a pair of nunchaku made of bones. Bonee threw the nunchaku, which spun around Jirachi's body and bound it. Due to interference from Guile Hideout, the capture fails.
Monlee is a gentle and caring Pokémon, best shown when he took care of Crystal after she broke her arms. He is a very stubborn Pokémon when it comes to battling and refuses to give up. He is Crystal's go to Pokémon when she needs a lot of power and has survived fights against powerful Legendary Pokémon such as Suicune, Registeel, and Jirachi.