Monkey D. Nikku is the Uncle to Monkey D. Luffy, Son of Monkey D. Garp, Brother to Monkey D. Dragon, Father to Monkey D. Raul, and Great Uncle to Monkey D. Luffy Jr.. He is a man with a former bounty of 682,000,000 beli because of his mastery of Armament Haki.
As stated before, he can use Armament Haki. Nothing else though.
Nikku's use of Six Powers is very limited, but still notable.
Age & Swordsmanship
As of now, he is a retired Pirate at the age of 67. He is also a Swordsman with the Former Title of Greatest Swordsman, until the record was beaten by Dracule Mihawk, Zoro, and Kure S. Akira.