I played this game 3 years ago, and recently played it all the way through again. For some reason I found it tougher this time around (I'm older and slower), but I enjoyed it both times. Although the story isn't anything special, and the characters rather stock, the main hero, Shin, is unusual: a half demon half human youth teetering on the edge of being overcome by some mysterious inner beast/spirit/emotion that can turn him into a killing machine. He is haunted by a past in which he slew all the inhabitants of his home village, so his powers come at great cost. He doesn't actually transform, as this is a story element, not a play feature. Demons are enemies of humans (demons eat humans), and half-demons aren't usually welcome anywhere. Shin and his companions must navigate between the two species amid a world always on the brink of war.
Battles, not story, are the heart of the game, and because the frequency of random monsters is great, and you are underpowered much of the game, everything connected to battle mechanics is important. Fortunately, there are several aspects to battles under your control, and they are all significant. That is what I like about Deva: your choices do matter, even though you will quickly learn the tactics that should lead to triumph.
There are no weapons as such in the game; equipment is needed to raise your defenses. You earn points to assign to 5 categories: attack strength, magic ability, accuracy, critical, and "lock" (adjust the decreases caused by movement). You learn a skill about every 10 levels (but you probably will not get above level 30 unless you grind). You don't have an unlimited number of points, and all the categories are of course important, so how you configure your party makes a difference. The good news is that for a modest amount of points you can reassign a character's points, and that may be necessary if you decide that magic is more important than physical attack (for example).
You also buy and find "orbs" that can be equipped to add additional skills. Although there are many different types of orbs available, a character can only equip at most 4 at a time (but you can have a larger supply in your inventory). These provide the typical buffs, debuffs, "all" attack/heal, and various elemental attacks. These can be expensive: a critical partial protection against sudden death costs 6000 units, and even battles in the very last dungeon only yield around 200 units!
You wind up with 5 characters, but only 3 can participate in a battle. Unfortunately, those who are "on the bench" do not share in the experience rewards, so it is likely somebody will stay at a low level. Does that matter? Yes and no: at one point a principal character goes missing, and then you must choose one from your "bench" to become a regular party member and share in the leveling up. It doesn't matter from a play perspective which you choose, although one of the characters has a more prominent role in the story. However, when your missing character returns, you may be faced with a choice about which character will return to the sidelines, or whether you wish to invest the time to raise the level of the missing character.
Cash is scarce in the game, and until late you will not find much useful equipment in the treasure chests. There are useful "battle arts" (consumables) in chests and shops that can help you save SP/HP, and these are varied: scarecrow, bombs of various powers, reflecting spikes, etc. Every 30 minutes (real time) you can get one random reward which is usually a consumable or some type of point.
The in-app shop has at least 3 useful items, two of which (the experience booster and the cash booster) are not otherwise available. You gain points for the store through battles, but it is a very slow process.
You will rarely be able to "auto" battles, and almost never in the latter part of the game. Some of this stems from the "move" feature of battles: both monsters and characters can move around an 8 square grid each turn. Movement increases evasion, but decreases accuracy and strength of the attack (there is a character attribute that can be adjusted to reduce this decrease). This is a bit of a pain, as it slows down the battle sequence, but it does present the possibility that if you choose an attack that is position dependent, and the monster moves away.....
You can flee every dungeon, and once you have the "teleport" type ability, you can instantly transfer to any previously visited location on the world map. So if you are struggling through a dungeon, you can escape, instantly warp to a village, and for a modest amount get restored. Most dungeons have a single restore point just before the dungeon boss. The dungeons themselves don't have special obstacles, but the constant attacks you sustain as you search for the right path do take their toll.
The interface is very good--most actions are possible with a minimum of menu tree tracing, and you can speed up the battle animations. The game is wordy with a few long scenes. There are only a few (6??) subquests, but some of them yield very valuable equipment. There is no difficulty option.
The post-game play consists of one maze (of a type already encountered) and another battle with the same final boss (who is somewhat enhanced). It is a long battle against the final boss, but not difficult. One thing I liked about it is that I was successful both under leveled, but using battle arts to preserve my SP, and at high levels where resources didn't really matter. Beware, however: even at high levels, there are monsters who move first and have instant death attacks!