gandalf
I think Frodo met Gandalf through his uncle, Bilbo Baggins. Though I think he's known Gandalf for awhile, let alone the whole trilogy, because in the beginning of the movies he knows Gandalf ( for the fact he got excited, yelled his name and jumped into the carraige thing. ) Frodo seems to trust Gandalf alot, I guess. Gandalf thinks of him as a kid, of course. But Gandalf throughout the trilogy, tries as best he can to keep his hopes up for Frodo and Sam and the Ring. That's why he helps occupy the eye so Frodo and Sam can travel to Mount Doom without being noticed ( And also the Orc's attention o.0 If not, they would've definitely been noticed. ) He fights for Frodo and ends up going off a cliff, but luckily didn't die ( and until the next book or movie came out, people thought he died ), just turned into Gandalf the White. It was cool. :P They thought it was Saruman, when he first appeared again. Anyways.. o.o A thing that also points out they had some time of relationship is that he yelled out Gandalf's name at the end ( or towards it ) of ROTK, return of the King. He didn't say Aragorn's or Legolas's. I think they were just acquaintences.