Johnny

Johnny and Yosaku are old friends and former bounty hunting partners of Roronoa Zoro.

History
They first appear during the Baratie arc and stay in the story until the end of the Arlong arc, both times helping out the Straw Hats. They arrive on the scene after Usopp and Luffy unknowingly shoot at the tiny island they're on for target practice. The partners had to stop because Yosaku was suffering from scurvy, but Nami was able to diagnose and treat him.

Johnny and Yasaku appear later in a filler episode (Johnny styling a mohawk, and Yosaku with a full head of hair). Their story is extended a bit as Johnny reveals he became a bounty hunter to save people and be cool, just like the bounty hunter who saved him when he was a kid. Yosaku on the other hand, just seems to want an easy life with a lot of money and little hassle. They cash in when Zoro takes out a large bounty and, given they pay his tab at the bar he was at for his food, can keep the rest of the money for themselves as an apology for taking their 'head'.

Abilities and Powers
Johnny, like Yosaku, is a swordsmen. While Johnny and Yosaku may be stronger then a normal person, against someone like Zoro they wouldn't stand a chance. As Arlong put it after Johnny and Yosuke confronted him after hearing Nami's history, "They were far to weak to be part of Zoro's gang." (Arlong still thought that Zoro had come for the bounty on his head at the time.

It is still unknown if Johnny or Yasaku have any attacks that have been named, since you are only able to see Johnny fight Luffy for a very short amount of time before being easily beat by Luffy.