Yu Kaito was a good minor character on the show. He was a psychic with the power of taboo.
He was capable of taking the souls of anybody who violated his taboo rules.
Kurama was actually a school mate of his, and he was always jealous of Kurama for being smarter than him.
Kurama was definitely the smartest of the bunch, as he was the only person to keep his soul in his body.
They play a game where they can’t say certain sounds or lose their souls. Which roughly turned into a game of silence.
Kurama did a lot of interesting things to win the battle.
Ironically he was just working for Genkai, but Kurama betted on the fate of a funny face.
And he lost the grand battle of wits due to laughter.