Tuvok is the second officer and tactical officer of the USS Voyager. He is one of the few members of the crew who seems to occasionally have a clue, so naturally hardly anyone follows his advice. Tuvok is a Vulcan, so he supposedly can't lie (if Data's assessment of Vulcans is to be believed), but he still managed to infiltrate the Maquis and become first officer of Chakotay's ship somehow.
Relations with Neelix
Tuvok proved to be a loyal and at least somewhat competent officer for Captain Janeway. Unlike most of the crew, he seems to be one of the few (including Tom Paris) to not accept Neelix immediately with open arms. This, of course, meant that Neelix decided to make Tuvok his constant friendship project, much to the annoyance of Tuvok himself.
If there are any doubts about Tuvok's disdain for Neelix, when Tuvok wants to test his emotional self-control, his "go to" plan is to put himself in the same room as a holographic copy of Neelix. Their conflict comes to a head when, through a transporter 'accident', he and Neelix are merged into a gestalt called "Tuvix". Eventually, Tuvix was forcibly separated back into Tuvok and Neelix by Janeway.
Throughout the journey home, Tuvok was able put his investigative skills to work as the chief of security for Voyager. He was able to solve at least two murders. On the second one, however, he did make the mistake of mind melding with a homicidal psychopath. He was also able to not only detect a saboteur and spy on board the ship, but also to set up a successful operation to flush out the spy, despite the incompetent interference from "expert" investigative reporter, Neelix. When it was pointed out that allowing Neelix to continue his bumbling could be a way to get the spy to out himself, Tuvok was not concerned that a possible way of the spy doing so might be killing Neelix.
At the end of the series, one of the things that pushed a now Admiral Janeway to violate the Temporal Directive was his succumbing to the Vulcan equivalent of Alzheimer's disease.