The first Indian ruler who joined the Subsidiary Alliance was the Nizam of Hyderabad.

The Subsidiary Alliance was a policy introduced by Lord Wellesley, the Governor-General of British India, in the early 19th century. It aimed to establish British control and influence over princely states in India without directly annexing them.

Under the Subsidiary Alliance system, Indian rulers were required to accept a British subsidiary force stationed within their territories. They were also required to enter into a treaty that restricted their foreign policy, maintained British troops in their territory, and provided financial support for the maintenance of the subsidiary force. In return, the British offered protection and guaranteed the ruler`s internal sovereignty.

The Nizam of Hyderabad, Mir Osman Ali Khan, was the first Indian ruler to sign a Subsidiary Alliance with the British in 1798. This marked the beginning of the implementation of the policy, which later expanded to other princely states in India.

Therefore, the correct answer is: the Nizam of Hyderabad.