Who is Jeremiah Brooks from The Warrior Heir and what is their importance?

Jeremiah Brooks is the warrior who is summoned by Hastings to act as a teaching tool for Jack. When Jack questions Jeremiah, he reveals that he fought his first battle at the age of fourteen. Afterward, he went to live with the Shawnee until he was captured by wizards. Jeremiah tells Jack that he suspects the only thing they have in common is that Hastings has them by their 'privates'.