I don't think things need to be necessarily that hidden. I doubt there's one specific known or unknown guy covertly running him.
Rather, Biden employs a great number of people in a variety of roles and interacts with (or his executive branch interacts with) who set or influence policy in one or more areas. These people aren't secret but range from middling-high profile (VP, Cabinet Secretaries, Department Heads, White House Chief of Staff, etc.) to low profile (White House staff in general, Speechwriters, Lobbyists, Interns, etc.) To some extent, all presidencies work like this. No president has been without advisors.
Biden is merely at the extreme end of having his policy set by others, likely I think to the extent that he has few if any policies which he is the driving force behind. Firstly, because of his age and declining mental state. Secondly, because he's old enough that his first reaction to some issues is likely out of step with the current democratic party, and to some extent I suspect he knows this and defers to avoid making political missteps, because he cares more about good politics than good policy.