The Heart Kingdom 「ハート王国 Hāto Ōkoku」 is a country bordering the Clover Kingdom. It is the home of Undine, the spirit of water, who formed a covenant with the country's rulers. The current ruler is Princess Lolopechka, and she has entered into an alliance with the Clover Kingdom to invade the Spade Kingdom.
The Heart Kingdom lies to the south of the Clover Kingdom. This country of green and water is rich with resources. Generations of princesses have used the power of Undine to protect the country. With a Mana Zone that extends over the entire country, the Princess herself can monitor everything going on in the country and even assist in sustaining its forests and crops by using her Water Magic and guards the kingdoms borders with powerful Trap Magic. Nobles of the Clover Kingdom are allowed to submit requests for entry and studying the country's magic.
The kingdom has its own system for rating a mage's skill and talent. It is divided into ten Magic Stages, with Stage Zero at the top and Ninth Stage at the bottom. The Kingdom itself possesses five Spirit Guardians who are ranked as Stage Zero.
Over the centuries, the citizens have adapted to the country's strong natural mana and developed a unique magic technique to channel that mana into spells, known as Mana Method. Since it uses natural mana, the technique is limited to magic attributes that can be found in nature, such as the four major elements and plant-based attributes.
- Royal Palace: The Heart Kingdom's Royal Palace is situated atop the highest mountain in the kingdom. Much of it is overgrown with vegetation, and a long staircase leads up to it.
- Elmora Lake: This underground lake is used for bathing.
- Polnfrume Forest: This forest is full of delicious fruits and various animals. The forest is burned during Vanica Zogratis's invasion of the kingdom.
A group of Magic Knights visits the Heart Kingdom as part of an investigation into devils. After testing the Knights' abilities, the Heart Kingdom forms an alliance with the Clover Kingdom to combat the Spade Kingdom, and the Spirit Guardians spend six months training several Magic Knights.
With five Dark Disciples, Vanica returns to the Heart Kingdom to fight and kill Lolopechka. The Dark Disciples defeat most of the Spirit Guardians. Several Magic Knights whom the Spirit Guardians trained, help combat the Dark Disciples. In the end, Vanica decides to take the Princess hostage instead and detonates the devil power in her Disciples, creating massive explosions across the Heart Kingdom. The Magic Knights manage to deflect enough of the blasts to survive and protect any nearby citizens. Shortly after, a group of elves arrive and take the injured Magic Knights back to Elysia.
- The Heart, Clover, Diamond, and Spade Kingdoms make up the four French suits of playing cards.
- In the manga, the ruler's title changes from "princess" 「王女 ōjo」 to "queen" 「女王 joō」 and then back to "princess". Since then, Lolopechka has been consistently referred to as a princess.