If you're paying attention, you can find references in the game to characters that should be familiar if you played past GTA titles.

Brucie Kibbutz returns on Bleeper, GTA's equivalent to Twitter.

Niko Bellic's old friend Packie McReary returns. Franklin can help him during a heist, and he'll be part of your crew for the rest of the game.

Niko Bellic is mentioned during the first heist, described as an 'Eastern-European' that was making a name for himself back in Liberty City and got out of the business.