Does anyone have some precise ideas of how SKXS's battle system works? I've picked up the game again recently and decided to try and make a table type for it, but it's been barely 5 battles and I'm already completely stumped.
Situation is simple: I'm in an early game situation, with all monsters in my party (Mechanical Bird [Buck's partner monster], Biting Fruit and Tree Spirit)  knowing the move "Hit" (that default Fighting-type move all early-game monsters seem to have).
I encounter the flying-type blue unicorn thing on the route between Tech City and Crystal Village (which also knows Hit). Then I use Hit with Mechanical Bird, the game displays a message translating as something like "It's super effective". Okay, why not. The unicorn monster also uses it on my monster. The game displays something along the lines of "It's not effective". Okay, weird, but maybe the game displays different strings depending on whether it's a player attack or enemy attack.
So I switch to the Tree Spirit, use Hit again, and this time "It's normally effective" gets displayed. Now I'm starting to get confused. Is there some kind of STAB like in Pokémon? Does effectiveness change depending on which type uses it? Are there hidden "attribute affinities"? So for a last test, I switch to the Biting Fruit (which is the same type as the Tree Spirit) and uses Hit again. And yet another different message gets displayed ("It's pretty effective" or something).

So what is going on? Are there some kind of hidden double-types for each monster that affect skill effectiveness? It doesn't appear to be random, each message is consistent when repeating the attack with the same monster. Is it related to the amount of HP the opponent loses? (I don't think that's it, since I once encountered the water type "Gurgle Demon" thing, removed over 50% of its HP with a Neutral-type attack (Wing Beat) and the game still displayed the "Not effective" message).
I'm kind of lost right now