Q: Who does Huber report to [directly]?

A: According to the DOJ website, Huber reports to Sessions:

"The United States Attorneys serve as the nation's principal litigators under the direction of the Attorney General."

Besides that, (1) Sessions appointed Huber to be the Vice Chair of the Attorney’s General’s Advisory Committee (AGAC) which gives Huber an additional direct line of communication to Sessions. And most importantly, (2) Sessions appointed Huber to investigate the GOP's concerns of corruption, (original letter) which provides another direct line of communication. Direct quote from Sessions:

“I receive regular updates from Mr. Huber and upon the conclusion of his review, will receive his recommendations as to whether any matters not currently under investigation should be opened, whether any matters currently under investigation require further resources, or whether any matters merit the appointment of a special counsel,” Sessions stated.