Who was the only Kaurava to fight on the Pandava side?

Yuyutsu was the only Kaurava to fight on the Pandava side.

Yuyutsu was a son of Dhritarashtra, the Kuru king, but his mother was a woman from outside Gandhari’s marriage. This made him a paternal half-brother of Duryodhana, Dushasana, Vikarna and the other traditionally counted Kaurava brothers.

Unlike most of his brothers, Yuyutsu repeatedly objected to Duryodhana’s unjust schemes and maintained sympathy for the Pandavas. Before the Kurukshetra War began, Yudhishthira announced that anyone who wished to change sides could do so. Yuyutsu accepted the invitation and joined the Pandava army.

He survived the war, while the Kaurava brothers who remained loyal to Duryodhana were killed. Afterward, Yuyutsu helped protect the Kuru kingdom and served the surviving royal family. The question refers to allegiance during the war: Yuyutsu was born into the Kaurava family but fought for the Pandavas.