Shaymin first appeared in its Land Forme in A Repeat of History! In a flashback, it was seen revitalising the barren wasteland that would eventually become Floaroma Town, leaving the people of the town forever grateful to it.
In the present, Shaymin appeared when Team Galactic began burning the flower garden. It attempted to stop their plans, but ended up being captured and taken to their ship, with Lewis and the others in pursuit.
In the following episode, Shaymin was experimented on by Team Galactic's researcher Charon in an attempt to understand how it was able to transport itself between dimensions. Eventually, Lewis and Nate managed to rescue Shaymin and it transported itself, Lewis, Nate and Bianca to its home - the Flower Paradise.
Using the Gracidea flowers blooming in the Paradise, Shaymin was able to transform into its Sky Forme. Returning to Sinnoh, Shaymin used its stronge Forme to battle Mars and the Team Galactic Grunts, eventually defeating them with Seed Flare. After Team Galactic escaped, Shaymin revitalised Floaroma Town once again and returned to Flower Paradise in order to recover its strength, saying farewell to the group.
In its Land Forme, Shaymin is rather shy and timid, as seen when it was scared of Lewis and Nate when they attempted to rescue it from Charon. Despite this, Shaymin is a very determined Pokémon, shown when it arrived to defend Floaroma Town despite its fear.
In contrast, while in its Sky Forme, Shaymin is much more confident and outgoing. It also has a playful side, as shown when it began playing with Aipom and Tepig after stopping Team Galactic. In this Forme, Shaymin is much more skilled in battle, being able to take down the Grunts' Pokémon without much trouble.