Balin is mentioned a couple of times in the book as an eventual option for players as a pre-generated character, but his sheet is missing from the collection at the back of the book, and isn't mentioned in the table of contents. I didn't see it elsewhere in the book and am hoping it is, or will be, available.
Balin is not a playable character but Patron. When he is "unlocked" (pg. 69, Shire Adventures), the Company may select him as their patron and gain Balin-specific bonuses (pg. 16-17 and pg. 210-211, Core Rulebook).
Wording under "Using the pre-generated characters" on pg. 58 is wrong/confusing.
As the story unfolds, two new heroes will become available as Player-heroes. The Loremaster is encouraged to keep these new heroes a secret until they become available, so as to not spoil the surprise for the players. These additional pregenerated characters are the Dwarf Balin, son of Fundin, and Bilbo Baggins himself!