Monkey D. Tiger (モンキー・D・タイガー, Monkī D. Taigā) alias
Pirate Adventurer (冒険家海賊, Bōkenka Kaizoku) is the father of Monkey D. Luffy and Portgaz D. Ace, younger brother of the infamous Revolutionary leader, Monkey D. Dragon, and the son of Monkey D. Garp. He is the pirate who inspired his sons to go on his journey and the captain of the Spade Pirates. The World Government fears him much as Whitebeard, they gave him the reputation as The World's Most-Dangerous Pirate (世界最悪の海賊, Sekai Saiaku no Kaizoku). He found the Gum-Gum Fruit which was accidentally consumed by Luffy. He is also one of the World Emperors.
Tiger is always wears a captain's coat over his shoulders, black haired, and, until Luffy was born, wore a straw hat obtained from his mother before she died while he was younger that he had worn for many years beforehand, and of which was synonymous with his image. He wears wore long light black pants and a pair of sandals and is unshaven with a short mustache. In all his appearances so far as captain of his own crew, Tiger has worn an open buttoned black shirt, leaving his X-shaped scar covering most of his chest exposed, which were revealed to have been given to him by Blackbeard.
The most striking thing about Tiger is he looks a lot like his sons, Ace and Luffy.
This is probably one of several theories that went to everyone's minds in the past when it was not yet revealed who's the father of Ace and Luffy.