Entei is first encountered in Mystical Forest, looking for Celebi. He meets the player, their partner, and the Expedition Society's Archen near Latios and Latias, who have recently been turned to stone. He suspects them of being the culprits of turning others to stone, but Archen tells him that he is part of the Expedition Society. Entei disappears after telling them that the next time they meet, he will not be merciful.
The Expedition Society soon travels to Fire Island Volcano, which is Entei's domain. Ampharos suspects Entei of being the culprit, and all but the player and their partner know that this is the true reason for their trip. Buizel wants to battle and defeat Entei, although Ampharos tells him that he is no match for Entei. The player, their partner, and Buizel make it to the summit of Fire Island Volcano, where Entei appears. Buizel battles him, but is quickly defeated. Entei tells him that his type advantage was not enough to defeat him. The player and their partner then battle Entei, but are defeated as well. Entei reminds them that he said he would not be merciful and decides to kill them. He is about to kill the player when the player and their partner's Harmony Scarves shine. Entei is defeated by their final Evolutions, but gets back up. At that point, the rest of the Expedition Society appears and Ampharos intervenes, politely asking Entei's forgiveness and revealing that he suspected Entei. Entei disappears. After a break for lunch, the Expedition Society leaves Fire Island Volcano.
Later, Entei is issued a warning that he will be turned to stone on Showdown Mountain:
"Turned to Stone Warning The next target has been decided! The volcanic Entei, one of three Legendary Pokémon! Consider yourselves warned: I will turn Entei to stone! The place will be Showdown Mountain, on the Water Continent! All are welcome to bear witness... as long as they, too, are prepared to be turned to stone! That is all!"
Entei heads to Showdown Mountain and is turned to stone. He is later found by the Expedition Society.
Later, Raikou and Suicune appear at Revelation Mountain to avenge Entei's defeat, believing that Krookodile was responsible. However, along with the player and the Expedition Society, Raikou and Suicune get turned into stone at the hands of Nuzleaf and Yveltal.
Raikou, Entei, and Suicune meet in the Voidlands and make an oath to send the player back to the Pokémon world. They saved the player, their partner, and Mawile from Void Shadows. They told the three that they had saved that they might sacrifice them in order to escape from the Voidlands. However, in the end, the legendary beasts (along with Mawile) sacrificed themselves for the player and their partner.
After Dark Matter was destroyed, the legendary beasts came back to the Pokémon world.
Personalities and characteristics
All three are intimidating Pokémon, and know that their power is much greater than that of most other Pokémon. For instance, when the player and partner arrive on Fire Island Volcano, Entei's domain, Entei stated that he was not above killing those who trespassed. They are also calculating but wise, and sacrificed themselves for the player and their partner knowing that they were needed in defeating Dark Matter. Also, Celebi reveals that Entei is more thoughtful than he appears, as Entei had asked Celebi to investigate the events of the Pokémon turning into stone, worried that the world was in real danger.