Nice game. I can tell you´ve put real effort into this. I like the music and the dynamic between the 2 main characters, and their expressions as well. I hope you are able to make custom art for the other important (and semi-important) characters since it does ad more personality than the custom ones taken from RPG Maker.
I like the nostalgic feel from the game, but I am happy that random encounters are not present, since I do not miss those from old school gaming.
If you are not supposed to be able to gain levels in this game, I hope there will be other ways to permanently raise the stats (especially hp and mp) of our heroes, since managing hp and mp during the demo was a chore that sucked the fun out of the game, because your poll of hp and mp currently is so small.
The single target attack Reinie learned after defeating the slimeboss did not seem to do more damage than the single target attack she already knew, so I´m not sure what the point of the new attack was supposed to be. At least that what it seemed like when I tested it out. If the protagonists are supposed to get stronger after defeating each boss, I hope the new skills will be more fleshed out in new updates.
I must sadly say that I did not like boss fight at all. I tested every attack out to see if anything could stop her regeneration. It seemed like using Reine´s 3 hit random target attack did stop her regen… but why? I got no text like “Reinie´s wild attack prevented the slime for regrowing”, something like that. Instead, she just stopped regen at the end of the round, which leaves the player being uncertain about why the regen had stopped, since there could be other explanations like: maybe it just stopped after x amount of rounds or a random number of rounds that changed every time the player started the boss fight. Was the Slime boss´s comment on Reinie´s instinct supposed to be the hint, that flaying randomly was the right choice?
Maybe there were other hints in the combat text, but I think I easily could have missed that because the combat text goes by so fast. The current speed is fine against regular enemies, but an option to change the text speed would be nice (like a slower speed normally, but if you hold a button down, the text flies by as it currently does), when you are fighting enemies that require more attention, like a boss.
Even after figuring out how to stop the slime regen. I found the fight to be more of a dull chore, that a fun challenge. I used the mage´s defensive ability, attacked with sword art and magic and healed with potions (hp and mp) when needed, until I won. But the need for potion was high, especially the mana potions, because the current max poll is so low. That made the duration of the fight, unwelcomely long. That rotation quickly became unfun during boss fight.
If you plan on making the future boss fights like that (requiring a heavy use of potions) I think you are defeating the purpose, that not getting stronger from regular fights is supposed to have. Just grinding away at enemies that each give a drop that can be sold for 50g is really tedious when you are not getting anything else out of the fight. If the protagonists got exp and levels, like most RPGs, from regular fights, those fights will gradually be finished quicker, with less needs for rests, since the hp/mp poll will be bigger, which would make the grind for potions less tedious. If you are adamant about not gaining levels in this game, I strongly recommend that the protagonists starting health, and especially, mana poll get increased.
I am not saying that I dislike a game design where a character´s power is locked at a certain level, until you get past the boss. It can be great, since it forges you to work with the power and resources you currently have, without cheesing your way out of the fight through grinding until your characters are several levels above the intended level for the current boss. I am just saying, I don´t like the way you have done it currently. No matter how you want the “progression of power” to be in the game, I hope you consider what effects you wish for it to have on the gameplay.