Johnny

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

Personality
Johnny is happy-go-lucky, much like Yosaku. Johnny tends to be the more serious of the pair, though he acts silly around Yosaku from time to time. Johnny is also dimwitted to some extent. He seems to be greatly misinformed about many things, from "Hawk-Eyes" visiting the Baratie regularly when the actual story was about "Drunk-Eyes", to not knowing how to cure Yosaku (and not knowing what ailed him in the first place). He also has a cowardly side to him, as seen when he not only tied up Zoro to prevent him assaulting Arlong Park, but through his willingness to simply abandon him to the Fishmen to run off to safety with Usopp.

Abilities and Powers
Johnny, like Yosaku, is a swordsmen. While Johnny and Yosaku may be stronger than 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 Yosaku 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.

Anime Only
Johnny and Yosaku appear later in a filler episode (Episode 135), with Johnny styling a mohawk, and Yosaku having 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'.

Meeting the Straw Hats
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.