When Hulu was young his drunk father gave him a devil fruit to eat. Not knowing what it would do to him Hulu at the fruit and passed out. When Hulu woke up his dad revealed to him that he had eaten the Gum Gum fruit and that being a pirate would be futile as he would not be able to swim. Hulu began to cry and was so traumatized theat he suffered a memory relapse and forgot that he had even ate the fruit and the fact that he could not swim.
Reflexes: Hulu has amazing reflexes because of the Gum Gum fruit, he is still unaware he has these though and is in shocked when he sees his ability.
Tough skin: Another ability that Hulu does not know he has. This is also because of the devil fruit.
Ehanced speed: Hulu was born naturally fast and has always been aware of this however his spped increased when he ate the fruit.
Flexability: Because of the devil fruit Hulu is very flexible but has never tried to extend his body.