Mercury is a fictional character and an Amalgam Comics superhero, whose true debut was in JLX #1, though his (metafictional) debut in the Amalgam Universe was in JLA: The New Blood #2. He is a combination of DC Comics' Impulse and Marvel Comics' Quicksilver.
Character overview and history
Pietro Allen was born in a far-off alternate future, where he was plagued by metamutant-hunting Sentinels. As a teenager, he found a way to time travel, and made his way to the past when the Judgment League Avengers were recruiting new members called the New Blood. With the assistance of Apollo, Firebird, and Runaway, the group took down Sentinels that were pursuing Pietro. He became Mercury and joined the JLA.
After the Mariner was framed by Will Magnus for destroying some Roxxon oil tankers, Mercury went with his New Blood teammates under the wing of Mister X to join the new JLX team. Mercury stayed with the JLX until the Armageddon Agenda, an event where Will Magnus was given permission by the government to hunt metamutants with his Sentinels.
During the Armageddon Agenda, Mercury was one of the few metamutants able to escape death or imprisonment. With the help of the entity known as Iriskani, who came from the future, Mercury returned to the future from whence he came, evading being incarcerated by the Judgment League Avengers. As he was in the future, he did not show up again for the JLX's second roster, assembled by Amazon to stop Fin Fang Flame.