Which fictional character is Tolkien’s first published Middle-earth protagonist?

Bilbo Baggins is Tolkien’s first published Middle-earth protagonist. He leads The Hobbit, which appeared in 1937 before The Lord of the Rings introduced Frodo as its central Ring-bearer.

Bilbo begins as a respectable hobbit who prefers food, comfort, and predictable routines. Gandalf and the dwarves draw him into the expedition to reclaim the Lonely Mountain from the dragon Smaug. During the journey, Bilbo finds the One Ring in the tunnels beneath the Misty Mountains after meeting Gollum.

Bilbo later becomes an important link between the two major books. He gives the Ring to Frodo before leaving the Shire, and his earlier adventure supplies essential background for the later quest. Frodo is therefore the principal protagonist of The Lord of the Rings, but Bilbo is the first central Middle-earth hero Tolkien presented in a published novel.