Leo Quintum is the project director for P.R.O.J.E.C.T., a restructured DNA Project. Quintum directs various super science projects, such as nanonauts designed to explore the Yoctasphere, or massive Titan Voyagers designed to explore outer space in the long term. He is often accompanied by his assistant Agatha, who can "read" DNA like sheet music. Other workers of the P.R.O.J.E.C.T. include photosynthetic Bizarro drones. Beyond the scientific purposes of these projects. Quintum describes the overarching goal as creating a replacement for Superman should anything ever happen to him.
One of Quintum's experiments, attempting to land on the Sun using a spaceship called the Bradbury, backfires on Quintum when it is revealed that one of the ship's crew is a clone created by Lex Luthor for the purpose of blowing up the ship. Superman arrives, defeats the clone and saves the ship. However, this had been Luthor's plan all along- in doing so, Superman overloads his cellular structure, causing him to slowly die.
Quintum tests Superman's powers back at P.R.O.J.E.C.T.'s base on the moon, and discovers that his powers have increased beyond measure. Quintum breaks the news to Superman, and Superman requests he not tell anyone.
Quintum is next seen with Jimmy Olsen, who takes over as director of P.R.O.J.E.C.T. for a day as part of a newspaper column. Quintum takes the day off to make contact with living neon gases called the Electrokund.
After regaining control of P.R.O.J.E.C.T., an accident involving the Underverse endangers Quintum and Agatha, but they are saved by Superman. The two warn Superman of the coming of a large parasitic organism that comes to be known as Bizarro World. Following the invasion of the Bizarros, Quintum and his G-Men guards fend off the invaders until Superman forces them into retreat. Shortly after this, as Superman descends into the Underverse, he tells Lois Lane that Superman is dying of solar radiation poisoning, despite Superman's request that he not.
Quintum later dons the guise of Flamebird, borrowed on Superman's behalf, and descends into the Bottled City of Kandor to talk to the leaders of Kandor. He gives them the proposition that, although he has failed to find a way to enlarge them, he and Superman have decided to place the Bottled City on Mars, unbottled. Although they initially refuse, they agree in the end. On Mars, Superman entrusts Quintum with his entire eight billion character genetic code to him, along with instructions on how to mix his DNA with that of Lois Lane's.
After Superman transcends typical conscience and goes into the sun to fix it, Quintum reveals his latest project to Agatha: Superman Secondus, the genetic son of Superman and Lois Lane.