On Tree-ear's fourth day of work, Kang helps him unload the cart so that the orphan can finish work earlier. He didn't have to help Tree-ear and wouldn't benefit from it, so it was an unselfish action. This indicates he has a kind heart.
Kang is ambitious. The trait of ambition is neither good nor evil. When the royal commission comes to Ch'ulp'o to select a potter's work for the palace, Kang gets to work. He uses a new pottery technique to make vases with inlay. Although his technique is innovative and beautiful, Kang is known for being an impatient potter. Therefore, his pottery isn't as fine as Min's. He keeps his new technique secret from the rest of the potters in the village. This neither makes him nice nor not nice, but neutral. He was the one with the idea to apply inlay to ceramics, so he didn't have to share it with anyone.
Although Kang isn't under any obligation to share his new inlay technique with the other potters, it would have been nice if he had. Then Min and potters like him could have competed for the commission on an even playing field. Still, Kang is polite to everyone, and he does something nice for Tree-ear at the beginning of the novel. As far as we know, he is a nice person, but the book doesn't tell us much about his character or temperament.