Kira is a Goddess worshipped by the residents of Kairos and antagonistic force in War for the Overworld.
No actual background is provided about Kira as an individual, however in the cutscene before the opening mission, Mendechaus explains to the reviving Underlord Oberon that she is one of the obstacles in the way of the campaign, as she offers protection to Kairos, its people and its Emperor, Lucius.
Throughout the campaign, several Empire forces, particularly priestesses and Matriarchs, are shown to be devoted to the goddess Kira and possibly obtain their powers through her. It is unknown if they still receive these powers from her, or their new underlord, when corrupted in a Torture Chamber.
During the campaign, the Underlord Rhaskos is threatened by Kira and those who serve her by using an artifact called the Aum to limit his power and lead an invasion into his home Realm. Despite the best efforts of Oberon, she successfully destroys Rhaskos' final dungeon core and banishes him to the Aether.
Like the Priestess and Matriarchs, the Emperor Lucius openly voices his support of Kira when confronted, although he still voices that he is as a god himself and no gods shall stand above or beside him after absorbing the Aum.
When the underlord Oberon is defeated by Lucius in his home Realm, his consciousness awakens in a vortex similar to that of when he awoke in his home realm at the beginning of the game, although this vortex is blue. Instead of being greeted by Mendechaus as Underlord, Kira's voice is instead heard, saying, "Welcome, Overseer", suggesting that Underlords who are banished to the aether become this new being alongside Kira - Rhaskos even began having memories of it in his dying moments. Whether Overseers become Underlords at a fixed point is unknown.
It is likely that it was Kira's voice in the intro cutscene saying "Now go, Ove-" to Oberon before he is sent to Mendechaus, which would mean Oberon was originally her Overseer all along, and was somehow hijacked and corrupted by Mendechaus.
"How curious that the thread of your fate has led you here... Come, Underlord; bear witness to the snipping of your friend's string"
"Mendechaus, your plays have grown far too predictable. It's time to eliminate your pieces and wipe the board clean"
"A guiding light, and nothing more"