When Atelier 801 came in possession of two mice, a contest was held to decide the names of them. In the end, Elise and Papaille were chosen. However, Elise died (and Papaille is now dead, too), and this was reflected in-game - Elise used to sleep in the room while Papaille cried, but now Elise and Papaille do nothing other than stand still. This is likely because Transformice grew more popular and Atelier 801 did not want to imply any form of death in the game.
You can see their names on the scoreboard section.
It used to be possible for guest players to check their cheese and buy clothing by using Papaille, but he cannot be clicked anymore.
One moderator currently known can whisper to other mice using Papaille's name. This moderator told a few mice from the room Troll to come to room 801 and you would get a "surprise." This room has also been used as a "party room", some moderators choose to play music and converse with other players.
Papaille looked different; having a gray fur instead of a brown before an unknown update.
↑In the Valentine's Day 2014 in the news it says
"Take your date to another level. As Papaille and Elise did, ask your lover’s hand in marriage and share your love with the entire community. Click on the “marriage” button (available in the friendlist) to marry your soulmate."