Nines is a charismatic revolutionary who fights for those who need it. On more than one occasion he saves the player's life, first by intervening at the public execution of your 'sire' (whose fate you would've shared) and later upon your arrival in LA, when the Sabbat come to get payback.
His views are heavily coloured by the things he saw during the Great Depression... a deep loathing for those who would oppress others and profit at their expense.
When you seek him out at The Last Round he offers to give you advice and even teaches you to fight better.
As the plot thickens, Nines is framed for the murder of Alistair Grout and a "Blood Hunt" is called, making him a fugitive. He hides out in Griffith Park, which is where you next meet him when Prince LaCroix sends you with a peace offering.
But, this too is a setup and you are both forced to flee for your lives. Nines is attacked and falls. His fate remains unclear until the endgame...