There's no Holy element in this game. Instead, there's "Sea" and "Curse". Sea is actually sacred in this game so it's the "Holy". This also means that water-oriented spells like Spread, Maelstrom, Tidal Wave, and Aqua Laser are basically light-elemental spells, and thus Brilliant Lance, Judgment, and Ray (traditionally light spells) are sea.
At the very very end of the game, notice the colours of everyone's Eres.
The seven mortal party members all have the seven colours. Note Senel's - It's dark blue. This doesn't represent blue: It represents Indigo. In the rainbow? Indigo is between violet (Chloe) and blue (Shirley). This showcases the love triangle. So obviously, that's why Chloe has violet - not Jay who wears purple.
So why does Jay get the Green glowing Eres instead of the purple, and why not Grune? The line is ROYGBIV - Green is dead in the middle. That's his own arc: That he's surrounded by friends and found family between the party and the Oresoren.
Grune meanwhile? She doesn't have green: Hers is white. Why white? Because it's all the colours - whreas Schwartz has none. Because Black is the absence of colour, whereas most people assume it to be white.
Shirley has light blue - specifically Cyan. Why that? Sea colour.
So why does Shirley not have many "Sea" aligned artes except for Tidal Wave? Early on, she was allergic to seawater, which is revealed to be Nerifes. Considering when she learns Tidal Wave (in the game's final act), by that point they'll have communed with the Great Nerifes.
At the end, Grune grants the gift of summon spirits: Rem, Efreet, Celsius, Gnome, Volt, and Shadow. Seems odd to not include Udine, right? Wait a minute - there's already a water deity here: Nerifes.
The hidden door at the Ruins of Frozen Light. Yes, Jay could have just told the party about it, but there was a closed door behind the hidden door. Either way, they would have had to go through the Great Hollow and the Underground Lake to get inside the Ruins, due to the second door that was behind the hidden door. Seeing as they opened the second door from inside the Ruins, they couldn't have gotten in even if Jay told them about the hidden door.
If the Ferines can live in the ocean, why bother living on land at all?
The sea had been extremely rough ever since the Orerines created the first landmasses. Presumably, it was too rough for the Ferines to live in.
When facing the final boss, you'll notice that you're facing the "Raging" Nerifes itself, an entity comprised entirely of water, not the possessed Mauritz. As for how he suddenly became good, the "Quiet" Nerifes may have aided in quelling his animosity now that the Raging Nerifes had been beaten into submission.
Another possibility is that during the second half of the game, it's shown that a decent amount of time had passed. During that time, the Ferines isolated themselves from the Orerines. One can presume that Mauritz did a considerable amount of thinking during that time, with possible help from Shirley. Shirley being a go-between for the two parties may have allowed some of the animosity to wither. It also helps that both she and Senel knew what kind of man Mauritz was before all of this happened.
The Party never states their personal feelings about Maurits after defeating him and it's clear the Ferines still rally around him as a political leader. Attempting to incarcerate or execute him is likely to increase hostilities with the Ferines. (Although just how organized, centralized, heterogeneous etc. Ferines society is remains unclear)
How do the heck Orerines able to use eres? Since the Raging Nerifes is the source of that power and Orerines are 1. Are not natives of the Shining Blue, 2. They are the ones who terraformed the planet which changed the planet forever and killed many Ferines in the process and 3. Orerines are killing and discriminating against its servants/worshipers to this day., why would Nerifes grant them the use of eres if they cause so much trouble for it and the planet? Was it a mistake? Did it hope that Orerines would wipes themselves out with the power? Are all the Orerines erens are somehow descendents of interbreeding between a Orerines and a Ferines? Why?
I believe it's mentioned somewhere that Eres only appeared among Orerines within the last fifty years. So there is definitely reason to believe the interbreeding theory. Though considering the state of relations between the two groups, that has its own uncomfortable implications.
An alternate theory is that the "Quiet Nerifes" granted the Oerines the power in hope that they would calm its rage.
Why are only Ferines able to fire the Legacy's weapons? It an Orerines ship. Presumably it had weapons even way before the Orerines had contact with the Ferines.
Fridge Horror: Chances are... the weapons were made after they landed... and adapted to use the Eres of the Ferines. The Ferines obviously decide that the end justifies the means.
The Nerifes Cannon and Wings of Light both use the Ferines as a power source, so it's likely they were either built or heavily modified after the Orerines landed on Shining Blue. Since the Ferines power the weapons, and the ship's systems had effectively been powered and controlled by a Ferines for three years prior to the start of the game, it makes sense that they can also use them.
