Does it have to be one of the 5 Lead Maintainers, or who can merge a pull request to the Bitcoin Core master branch?
Whether a pull request is merged into Bitcoin Core rests with the project merge maintainers.
Maintainers will take into consideration if a patch is in line with the general principles of the project; meets the minimum standards for inclusion; and will judge the general consensus of contributors.
Any maintainer can approve merges. See Is there a list of core Bitcoin committers? for a list of maintainers.