What was the name of the Buddha's cousin who tried to kill him?

The Buddha’s cousin who tried to kill him was Devadatta.

Devadatta was traditionally related to Gautama Buddha as a cousin and, in some accounts, a brother-in-law. He became a monk in the Buddha’s community and was initially respected. Buddhist narratives later portray him as becoming jealous of the Buddha’s authority and seeking control over the Sangha.

The traditions describe several attempts on the Buddha’s life. Devadatta is said to have arranged for assassins to attack him, rolled a boulder down a mountain in his direction, and set the intoxicated elephant Nalagiri loose on him. The attacks failed: the assassins converted, the rock merely injured the Buddha, and Nalagiri was calmed by the Buddha’s loving-kindness.

Devadatta is also remembered for creating a schism. He proposed stricter compulsory rules for monks, including permanent forest residence and complete abstention from meat. When the Buddha declined to impose them, Devadatta formed a breakaway faction. Sariputta and Moggallana eventually persuaded many of those monks to return.

Suppabuddha was another relative associated with hostility in some traditions, but Devadatta is the canonical quiz answer for the cousin who attempted to kill the Buddha.