Ash's Unfezant

Ash's Tranquill is the first Pokémon that Ash obtained in the Unova region.

As a Pidove
Pidove was caught soon after Ash began travelling in Unova in BW002: Enter Iris and Axew. Ash tried to catch another Pidove beforehand, but the rest of the flock attacked himself and Pikachu and flew away, leaving this particular Pidove pecking at the ground. She proved herself to be a strong battler, holding her own against Pikachu before being knocked out by his Thunderbolt and caught by Ash.

Pidove was then sent out in an attempt to rescue Pikachu and Axew from capture by Team Rocket. However, it was easily defeated by Jessie and her newly-caught Woobat.

Pidove has mainly been used by Ash to search for other Pokémon, as seen when she was used to find two wild Darumaka in BW008: Saving Darmanitan From the Bell! and a wild Snivy in BW007: Snivy Plays Hard to Catch!. She played a major role in Snivy's capture, as she was unable to be affected by Snivy's Attract.

Pidove was used against Trip's Frillish in BW010: A Rival Battle for Club Champ!. She was quickly defeated by Frillish's Water Pulse. She was later sent to Professor Juniper's lab in BW018 to make room for Ash's Sewaddle, but was returned to his party in BW021.

As a Tranquill
In BW022, she helped clear out a swarm of Venipede from Castelia City. She was nearly hit by a Sludge Bomb, but was saved by Trip's Tranquill. She then cleared a few more Venipede and suddenly evolved into a Tranquill.

As of BW025, Tranquill was sent back to Professor Juniper's lab while Swadloon stayed on Ash's team but later in BW032 Ash Brought it back.

Mystery Egg event
A Japanese event in December 2010 and at Toys R Us in the United States in May 2011 randomly distributed in-game representations of Cilan's Pansage, Ash's Pidove and Iris' Axew. Pidove is always female and has the ability Super Luck

Trivia

 * Tranquill's ability was revealed in a giveaway of eggs containing the three main character's Pokémon.
 * Tranquil is the first pokemon Ash caught in the Unova region.
 * It hasn't been in a gym battle yet.