Eric

Erik the Whirlwind (つむじかぜのエリック Tsumujikaze no Erikku) is a mercenary in One Piece. He is a non canon character, appearing in the anime only, being the main antagonist of the Warship Island arc. Here he is hired by Commodore Nelson to capture the famed Thousand-Year Dragon, a creature whose bones are said to be the source of eternal youth. However, Erik was only using Nelson to find the Dragon for himself for power.

Abilities
Erik has show to be quite acrobatic, jumping to evade Monkey D. Luffy's attacks.

Devil Fruit
Wielding the power of the Kama Kama no Mi, Erik is able to use his lengthened fingernails to cut through the very air to create razor-sharp gusts of varying size. Erik proves himself to be a dangerous opponent when ultimately facing Luffy, inflicting multiple wounds against his opponent, but he proves to be no match for the Straw Hat captain.

History
His part in the storyline is short, but rather humorous. As a mercenary under Nelson's command; he is ordered to retrieve the runaway girl Apis and capture the legendary Sennenryu, Ryuji. He tracks down and follows the Straw Hats throughout the entire Warship Island arc, eventually betraying and (supposedly) killing Nelson and fighting Luffy for the Dragons and their mystical "elixir". He manages to deal a significant amount of damage to Luffy, but is defeated when Luffy sends him flying of the Lost Island with his Gomu Gomu no Bazooka attack. Erik later reappeared once again to obtain his revenge just as the crew began to climb the Reverse Mountain, leading to the infamous Grand Line, but found himself tipped over the edge of the Going Merry and into the water by the wily thief Nami before a battle could erupt. He is most likely dead, due to the negative side effect of his Devil Fruit and the strong currents of the mountain.

Translation and Dub Issues
A running gag throughout that arc was Luffy and the others would mishear the name of his Devil Fruit (Kama Kama) as Okama Okama, making them think Erik was a crossdresser. In the FUNimation dub, since the name of his Devil Fruit was changed to the "Sickle-Sickle Fruit", the joke was changed to them mishearing the name as the "Sicko-Sicko Fruit".

Trivia

 * This Devil Fruit gives Erik power similar to CP9's Rokushiki move Rankyaku but, as Erik is a filler character, it is unknown which is stronger. Erik's abilities comes from a Devil Fruit power, but the Rokushiki moves are of Super-Human Strength. Luffy was able to withstand Erik's attack with ease, while he recoils after being hit with Rankyaku.