I really like this game. I hope it gets updated soon.
I guess...
-Garnet look so similar to quest of kai character(made me think both of them is the same developer)
I'm pretty sure it's not the same developer, but Garnet definitely seems to be based on or inspired by Kai.
-fighting require very expensive maintenance , grooming a horse is the better way for earning maybe.
It's really easy to make money in this game. Whoring yourself out can make you lots of money. (You can have sex with most of the characters in the first town, although not all of them give you money)
-goblin boss look like the massive troll with a pipe-bomb(tree trunk club that fire a fire ball)and Ifeel this fireball tosser troll is unbeatable.
He is certainly
not unbeatable. He does not have many hit points. You have to deal enough damage to kill him quickly. I was able to do this at level 15 with the knuckles you buy from Gai's dojo (200 attack power). It's probably possible to kill him earlier than level 15, since your attack power is largely based on your weapon, not level.
His fireball will one-shot you though, so be careful.
I can't figure out how to save the sister from the goblins
Neither can I. Even after I got the key from the goblin boss, I can't open the cell where I would expect her to be held. It's possible that mission isn't completed yet, but I'm not sure.