Technically, a version of Brynden Rivers did make it into the series as The Three-Eyed Raven who summons and later mentors Bran in the ways of greensight. However, the HBO series gave him so little screen time and development that he may as well be a different
person. In the show, he's just a sagely old man living with the last of the Children of the Forest. In comparison, the books make it clear that The Three-Eyed Raven is just a moniker and a title for the exiled Brynden Rivers. Brynden was a Targaryen bastard who grew up to become the Varys of his time before being exiled to the wall, where he climbed the ranks of the Night's Watch before becoming Lord Commander. A Dance with Dragons paints him as this fascinating figure with a sprawling history to his name but in Game of Thrones we don't even get that much. In fact, we get so little info that some fans have speculated that the show's version of the character will later be revealed as an older version of Bran who timetravels into the past. Still, Brynden Rivers is undoubtedly one of the coolest figures in the A Song of Ice and Fire series and it's a shame to see him not get the treatment he deserves on TV.