"[with] Mercy Wardwell so frustrated by her fifth fumble that she beat her head repeatedly against her broom handle...."
-- Ginny Weasley reporting for the Daily Prophet, 4 July 2014 (Pm)
Mercy Wardwell was a Chaser for the United States National Team at the 2014 Quidditch World Cup.
Their next win was against the Liechtenstein side. After the match over-exuberant American fans kidnapped Lichenstein's team mascot, causing an international incident.
The playoff for third place was against the Japan National Team. The Japanese Seeker caught the Snitch directly underneath Wardwell's foot to end the match (330 points to 120), which meant they ended as fourth overall in the tournament (Pm).
Other canon notes and references
She was likely to have been named after (and was possibly distantly related to) Mercy Wardwell, a 19-year old who was accused of witchcraft during the Salem Witch Trials in Massachusetts. Her father, Samuel Wardwell, was executed but she and her mother Sarah survived (University of Virginia and Wikipedia).
Mercy means "compassion, forgiveness, pity" and derives from the Latin "merces" (payment, reward) (Babyname Wizard).
Wardwell is an English surname of Norman origin meaning "guardian, watchman" (House of Names).
As she is from the USA, Wardwell may have attended the Ilvermorny School of Witchcraft and Wizardry.
From the Web
Screenshots of the Daily Prophet's coverage of the 2014 Quidditch World Cup: http://imgur.com/a/AXutv