Cool game! I like the feel, the art, and the sound but I also have some suggestions.
1 - You don't need to add "Inner Thoughts" to your inner thoughts. It's clear enough.
2 - You should add a hint to where to find your sister. Her corridor looked literally like any other.
3 - You desperately need a map. I gave up on the second dungeon after wandering through the same corridors several times.
4 - The story is super light as is the combat mechanics. Either pick 1 to be the focus of the game or both.
5 - When I buy spells there is no description of what they do. There is no tutorial or any information about the different status effects and damage types.
6 - There is no rest, no camp or town. You could expand that location with the skill purchase to have more stuff going on.
7 - The tension in the game should be a roller coaster with ups and downs. After the boss battle, there should be some lull. Some story, rest or respite. Instead, it just goes straight into more of the same random combat and dungeon-crawling loop.
8 - As mentioned before combat mechanics is quite shallow. For the first boss, all I did pretty much was just spam normal attacks. For instance, if you have a double ATB, one to select an action and another to implement it you should let the second delay take different lengths depending on your action and possibly let you cancel opponents' turns like some similar RPGs have (Grandia for instance). You should probably also have a list showing turn order to help the player plan around that.