After forty years! Some translations say "four years" - probably to remove the obvious absurdity.
David's counselor, Ahithophel (15:12)
This is the first mention of Ahithophel in the Bible. He was David's counselor but left him for Absalom. Wikipedia: Ahithophel
Then Hushai came (15:32)
This is the first time Hushai is mentioned in the Bible. He will be one of David's spies on Absalom's conspiracy. Wikipedia: Hushai
Zadok and Abiathar (15:33)
Zadok and Abiathar were priests. This is also the first time that Zadok is mentioned in the Bible. Abiathar was the son of Ahimelech and was the only priest to escape Saul's massacre of Nob. (1 Samuel 22:6-23, Episode 112)
Wikipedia: Zadok, Abiathar