Lumiaris

Lumiaris is the goddess of light and Skullhell's nemesis, the 2 of them having been waging war on each other since time immemorial. However, according to Darunggar's Legends the 2 use to be one as Enos (literally: one god), but one day that god split into 2 conflicting half.

Appearance
On a tablets found in Darrungar, where Skullhell and Lumiaris once fought, she is depicted as a naked long-haired woman with a rainbow on her back. For weapon she is shown to use two sphere one big and one small, whether those are weapon or a representation of magic is unknown.

According to Merry, in Grimgar, Lumiaris is always just represented by a hexagram, never drawn.

Personality
Mostly unknown, but since she did destroy Darrungar in her fight with Skullhell, before abandoning that world to continue the fight in Grimgar leaving numerous followers behind. She probably isn't as good as the title of Goddess of light may lead some to believe.

Also she, for some reason, forbid her paladins to heal themselves lest she took away their blessing.

Darunggar's legend
According to Darunggar's legends, Lumiaris and Skullhell were once one and the same as Enos (one god). At that time Darunggar was called Fanangar (paradise), but after Enos split, the 2 nemesis gods gathered massive massive armies to wedges wars on each other. Eventually, the whole world was at war and became Jidgar (field of battle). Finally, the battle between the gods came to an end, how or why now lost to time, but both gods left for Grimgar leaving behind many of their followers in a now-destroyed world that finally took its current name: Darunggar (the land of despair), a godless world.

Considering the state of Darunggar, the fact that that legend seems to be widely known, the way divine magic works there, and the fact that there's some archeological evidence to back it up; it seems very likely that that legend is mostly true.

Grimgar's legend
In the myth of Arabankia, a story told since ancient times about a paradise-like land hidden somewhere in the world and a founding myth for (at least) the Kingdom of Arabakia, it's said that Grimgar was once laid waste in the terrifying battle of the gods.

Unfortunately, no more is said about that war, so we don't know who the waring gods were, if they were native gods or not, how fierce the battle was, or how it stopped. Moreover, we're explicitly told that that story is just a legend, likely based on some truth but heavily modified, with many versions, and that it was even used for political means at least one time.

As such, that story of a great war between the gods could have originated from people fleeing the war-torn Darunggar, but over time, shift to take place in Grimgar as most people probably wouldn't know of Darunggar.

That said there's still the possibility that for a time after they fled Darunggar, Lumiaris and Skullhell restarted their war, in Grimgar this time, before stopping for some unknown reason.

Level. 9 Here and Now, to Far Far Away
While she is mentioned and characters have been using light magic granted by her as far back as Level. 1, Level. 9 is the first volume in which she visibly made an action. As, while she still doesn't appear, she does answer Tsuga's prayer making making fog cover the Thousand Valley.

Abilities
While we don't know the true extent of her ability, we do know that she is able to give every light magic user access to a small amount of her power.

And according to Darrungar's legend, her battle with Skullhell made the whole world a battleground, before finally breaking it, leaving that world in the state we see in the series.

Finally, the spell prayer doesn't have any fixed effects as Lumiaris is the one choosing what will happen or not. So from the event of Level. 9, we know that Lumiaris is capable of some form of climate manipulation, causing fog to suddenly cover the Thousand Valley when it would normally be exposed.

Skullhell
The 2 use to be one god named Enos which split into them long ago, in Darunggar. As god of opposite element (Darkness and Light) they seems to have been in conflict since the beginning, their war fierce enough to laid waste to world(s).

By the time of the story they're not waging open war anymore, and even allow volunteer soldier from both faith to work together. However, the Church of Lumiaris seems to be the official religion of the Kingdom of Arabakia and from what said Neal in Level. 15, the cult of Skullhell isn't allow in the mainland.

Trivia

 * Her name likely come from the latin world "lūmināris" the second-person singular present passive indicative of "lūminō" ,a verb meaning to bring light.