Does the President actually have the power to pardon and is the decision final?

President Ram Nath Kovind rejected the mercy petition filed by Mukesh Singh, one of the killers in the Nirbhaya case. He had sent a mercy petition to the President after the Supreme Court had rejected his curative plea. Let us take a look at the powers of the President in death sentence cases and whether the rejection of a mercy plea is a finality in this process.