Is a guardian required for a man at the time of the marriage contract, and can this guardian be any relative of his?

A man does not need a guardian (wali) at the time of concluding a marriage contract; he undertakes the contract himself. As for a woman, she needs a guardian, based on the Prophet's (peace be upon him) saying: "Any woman who marries without a guardian, her marriage is invalid, invalid, invalid." Guardianship over a man is only required if he is insane or mentally incapacitated.