The Radiant is the opposite of the Spectre. While Spectre embodies god's vengeance, Radiant is the embodiment of god's mercy.
Radiant was once Sister Clarice, a nun who was attacked and brutally beaten by three men. She was not angry at them, and was at peace when she later passed away at the hospital. When she died, she was fused with the Radiant and set out to bring mercy to the world.
Radiant was first seen during Final Crisis, saving Renee Montoya (the Question) from a disgruntled Spectre. Spectre, angered that God forced him to kill his own son but spared Question and Libra from his wrath, attacks Radiant but she is able to hold of against him. After an argument, they are attacked by minions of Darkseid and find out that they cannot release them from the Anti-Life Equation. They flee and take shelter along with other survivors in a nearby church. There, Radiant sees the men who killed her and her own faith begins to falter.
The church is then attacked by Vandal Savage reborn as Cain, who holds an ancient grudge against the Spectre for branding him. Cain uses the Spear of Destiny to separate Spectre from his host, Crispus Allen, and uses him to subjugate the world for Darkseid. Radiant, with some help from Question and Huntress, is able to protect the people inside the church.
She reveals that, because free will is God's greatest gift to man, it must be man, not heavenly beings, who will decide the battle for mankind's survival. Question understands this. She and Huntress attack Cain and try to take the Spear of Destiny from him. With the help of the three men, who sacrifice their lives and thus earn Radiant's forgiveness, Question manages to wrestle the Spear from Cain, purify it, and the give it to the Spectre who undoes everything Cain has done and banishes him away.
Following this, Radiant appears to have retreated back to limbo along with the Spectre. Later still, with the Spectre, Radiant attempts to offset the arrival of Mandrakk the Dark Monitor but is defeated and sucked dry of power.