Ninja Gaiden Easter Egg - Hayabusa Armour set, Combat Style and Ally - Rise of the Ronin
In order to unlock him you need to complete the mission: 'In Search of New Knowledge'.
You can simply progress the bond by using him in missions.
Bond level 1: Hayabusa-ryu Combat style
Bond level 2: Combined assassination + Combat style upgrade (Advanced)
Bond level 3: Gesture: Ninja pose + Combat style upgrade (Master)
Bond level 4: Full hayabusa set