Gollum is the creature who challenges Bilbo with riddles in J.R.R. Tolkien’s The Hobbit.
Bilbo encounters Gollum beneath the Misty Mountains after becoming separated from the Dwarves. Gollum lives beside an underground lake and possesses a mysterious golden ring. The two agree to a riddle contest, with Bilbo’s final question—asking what he has in his pocket—helping him escape after he finds the Ring.
In The Hobbit, Gollum is presented as a strange and isolated creature. The Lord of the Rings later gives him the name Sméagol and expands his history, revealing that the Ring corrupted him over a very long period.
Balin is one of Thorin’s Dwarves, Gríma Wormtongue is associated with Rohan, and the Necromancer is another name for Sauron. None of them is Bilbo’s riddle opponent.