Lego Monkie Kid is The Lego Group's first product line based on Chinese culture. Although The Lego Group had previously released Lego Chinese New Year sets, it was the first time that the company had released an entire product line based on a Chinese theme. Following research in China, The Lego Group discovered that the classic Chinese story of the Monkey King and Journey to the West is widely known within Chinese culture. As a result, the story was chosen as the main inspiration for Lego Monkie Kid. Simon Lucas, senior creative director commented, "It feels like it was in the DNA of everybody there. So we thought it was a great starting point to tell a new story that's rooted in Chinese culture". The Lego theme was developed over the course of two years and interpreted the story in a different way to the original. Lucas explained, "The people we spoke to have grown up with these stories, and we wanted to find out what's important to them, and how to interpret that in a new way...So, being 100% accurate to the original was not necessary, really, because we were picking up the key ingredients". The product line does, however, include recognizable characters from Journey to the West.

The designers behind Lego Monkie Kid revealed that researching Chinese stories and testing with children were important in the development of the theme. Lego Senior Design Manager Dennis Fong explained, "We felt that we hit something really big when we had this concept of the Monkey King ... Seeing all the energy from the kids, the parents, how they could instantly recognize it because we had a lot of key iconic elements in there. And, you know, we didn't need to tell a new story. We had the old story as a foundation for us to build upon."

The initial concept for the theme originated in a rough sketch created by Ned Rogers in December 2021. The sketch depicted several construction sets are transformed into animated vehicles and which parts of the design remain. Featured the concept art are Monkie Kid's Cloud Jet, Pigsy's Food Truck and the Tuk Tuk from Iron Bull Tank. With Lego Monkie Kid theme from The Lego Group was developed as an animated television series, the images showcase how the early set designs were taken and turned into animated models on screens.

The Lego Monkie Kid theme was launched on 16 May 2020. As part of the marketing campaign, The Lego Group released the eight sets inspired by Monkey King and Journey to the West. Each set featured different giant mecha, buildings and vehicles. Minifigures including Monkie Kid, Mei, Monkey King, Mo the cat, Sandy, Mr. Tang and Pigsy were released as well. The sets were designed primarily for children 6+.

• Monkie Kid ("MK" for short, 悟空小侠/齐小天, voiced by Jack DeSena ): The main protagonist of the series. A young noodle delivery boy chosen by Monkey King as his new successor to battle against the forces of evil, he wields Monkey King's golden staff and given almost all of his powers.
• Mei (龙小娇, voiced by Stephanie Sheh ): The descendant of the legendary White Horse Dragon and MK's best friend, she is gifted with a dragon aura and her family's jade dragon blade to fight against foes, she would later inherit Red Son's Samadhi Fire in season 3.
• Pigsy (朱大廚, voiced by Dave B. Mitchell ): The stern owner and head chef of Pigsy's Noodles who acts as a fatherly figure to MK. His weapon of choice is a rake.
• Sandy (沙大力, voiced by Patrick Seitz ): A river ogre who was formerly a rage-filled soldier, but became peaceful after going to therapy for his anger. When enraged, his eyes glow purple while his pearl necklace turns into orbs of fire. His weapon of choice is a crescent staff. He was also made into a minifigure since the Season 3 sets.
• Tang (唐師傅, voiced by David Chen ): A regular customer at Pigsy's Noodles, often telling MK stories about Monkey King in exchange for free noodles (which angers Pigsy for being cheap), it is revealed in season 3 that he has the power of chi manipulation, his weapon of choice is a khakkhara. He is believed to be another reincarnation of the Golden Cicada (金蟬子), who previously became Tang Sanzang.
• Monkey King (also known as "Sun Wukong", 孫悟空, voiced by Sean Schemmel ): The legendary hero who famously went on the Journey to the West and battled various demons, he then decided to retire on Flower Fruit Mountain and train MK as his new successor to fight against evil demons while also helping the team on occasions.
• Demon Bull King ("DBK" for short, 牛魔王, voiced by Steve Blum ): Princess Iron Fan's husband and Red Son's father, he is the leader of the Demon Bull Family and the main antagonist of season 1. Demon Bull King was defeated by Monkey King centuries ago and trapped inside a mountain for 500 years until he was freed by his family and plans to takeover Megapolis, his weapon is a battle axe, while also cannons and a flamethrower when transformed into a giant, he is made into a brick-built figure in "The Flaming Foundry" set.
• Princess Iron Fan (鐵扇公主, voiced by Gwendoline Yeo ): Demon Bull King's celestial wife and Red Son's mother. She was once a member of the royal Celestial Court, but later defected after falling in love with Demon Bull King and giving birth to Red Son, thus forming the Demon Bull Family who want to conquer Megapolis, the Celestial Realm and then the world. She is portrayed as cold-hearted and cruel who would do anything to please her husband, but despite that is also shown to actually care about her family. She wields a giant magic fan that can summon whirlwinds while also the power of aerokinesis.
• Red Son (紅孩兒, voiced by Kyle McCarley ): Demon Bull King and Princess Iron Fan's half-demon, half-celestial son. He is an egotistical mad scientist who builds diabolical weapons and has a literal hot temper when humiliated in front of his father, whom he seeks his affection. He would occasionally join MK's team briefly to help them stop much bigger threats. It turns out he is responsible for the creation of the three Samadhi Rings due to his power being unstable when he was born. He has P…

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