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Umbreon and Espeon are a brother and sister duo who are researching magnagates.
They discovered how to find and open a magnagate. The player and his or her partner help them at one point in Pokémon Mystery Dungeon: Gates to Infinity. They have discovered, with the player's help, that all magnagates have a boss on the top floor. They help the player and his or her partner on the first magnagate dungeon. They open the magnagate for the player whenever he or she goes into one. They give more information about magnagates when the player speaks to them.
When the duo is first shown in Gates to Infinity, only Umbreon appears, who is a mysterious figure running through a Forest and then Post Town. Umbreon eventually ends up getting injured, although it is mostly exhaustion that poses a threat to his health. He is then taken to Swanna's Shop to rest up. Later on, the Pokémon in Post town ask Umbreon why he went to the town, but couldn't provide answers yet. One Pokémon, Virizion, reveals she met Umbreon and Espeon once before at the Great Glacier and reveals they are Dungeon Researchers. The Partner Pokémon then wonders where Espeon is, but doesn't get an answer due to Umbreon being too weak.
The next day, Umbreon forces himself to get out of Post Town, but still needs to recover. When asked why he is rushing to leave the town, he reveals he was attacked by a group of Pokémon and lost contact with Espeon since they ended up splitting. He says they were attacked because they had Entercards, which has something to do with the mysteries of powers in certain areas. After making the Pokémon listening to the story fall asleep, he goes outside and shows them what the Entercards could do. When done with the demonstration, he reveals a Magnagate, a portal to a dungeon. He notes it is unfinished because the two other Entercards that are required are with Espeon. The Player's team then go and rescue Espeon from the hostile Pokémon and head towards the Crags of Lament.
When the player gets to the middle of the dungeon, Espeon is running from two Pokèmon, whom tell her to look for them. Espeon refused to go after them and continues running. Eventually, when the player reaches the end of the dungeon, Espeon gets surrounded by the bad Pokémon, who are revealed to be Purugly and Toxicroak. The duo gets Espeon cornered and takes her bag, although it is empty. The duo then demands Espeon to tell them where she put the Entercards. The player then arrives and battles Purugly and Toxicroak. The duo loses and flees from the player's team. Espeon walks slowly to the team with minor injuries. She thanks the player and and goes with them back to Post Town.
Upon arrival to Post Town, Umbreon and Espeon are reunited and thank everyone for their support. When they wonder why Purugly and Toxicroak would take the Entercards, they mention the thieves might go to the Great Glacier, surprising everyone in town. The duo then realizes they didn't explain their jobs yet. They tell everyone that they spent their careers studying the Great Glacier, because of the Great Crystal in deep Great Glacier. They plan to use the Entercards to create a dungeon leading beyond the crevasse. Umbreon states it might be too late because he thinks that Purugly and Toxicroak stole Espeon's Entercards. However, Espeon scolds him for jumping to conclusions and reveals that they are indeed still in safe keeping, but Espeon does regret doing what she did to save them.
Shortly after, a Swadloon goes to Espeon and hands her Entercards she gave to him for safe keeping. Espeon becomes glad Swadloon is safe, but then breaks down crying. Leavanny, Swadloon's mother asks why would her son would be in danger. Espeon reveals she was headed for Post town, but due to Purugly and Toxicroak always being one step ahead of her, she buried her items in Inflora Forest. Later on, Swadloon discovers the Entercards, causing Espeon to panic. Espeon decides to give him a red stone she buried with her Entercards if Swadloon kept an eye on the cards until she went to pick them up. Swadloon keeps his promise and Espeon leaves after hearing a rustle, suspecting it to be her pursuers. Shortly after, Espeon realizes that this also endangers Swadloon and heads back to Inflora Forest to correct her mistake. However, by the time she returned, Swadloon had already left.
Leavanny and Swadloon try to stop Espeon from crying because she had no choice but to leave the Entercards with Swadloon. Leavanny tells Espeon that she is proud of both her and Swadloon for keeping their word. She remains crying and Umbreon is relieved the Entercards are safe. Umbreon decides it is best for both of them to remain in Post Town since they are likely to get targeted again and they still need to recover from their injures. Swanna lets them stay in Post Town, but stresses that they'll still need to pay.
The next day, The duo meet up with Virizion on top of a hill. The Player's team see them and the duo tells them they feel a lot better thanks to everyone helping out, but they can't afford to waste any time because they are nearly complete with their research. The Partner Pokèmon asks if they can help out Espeon and Umbreon with their research. The duo is glad they want to help, but the night before, they decided to join the Player's team to repay the player and use their strength to achieve their dream. They also feel that since the Great Glacier is an unexplored dungeon, they might not be able to return back without them. The Player lets them join the team, but Umbreon and Espeon say they can't join right away because they are busy finishing their research. The Player's partner is okay with this, as it felt the same way. Everyone then continues with their day.
The duo will start the adventure with the following moveset:
For more information on this Pokémon's species, see Umbreon and Espeon.