Figured I'd throw my idea of Sykes as an actual submission instead of just in my head :p
Age: 12 years old (assuming Link is 11 or so)
Race: Human/Hylian/Regular, whatever, the same as Link :p
Home: Hyrule Slums, although when Link sets out on his travels, he does too.
You would find out a lot about Syke's past and personality from talking to other NPCs. Sykes, like Link, was orphaned at a young age. At the game's opening, he lives in Hyrule Slums with a number of other poor youth under the guidance of Faron. He lived on his own for a couple years before he found this surrogate family, which caused him to develop both excellent thievery skills and an insecure personality. Sykes trusts no one and always puts himself first, as both traits were necessary to survive on his own.
As one of the oldest children in the gang at this point, as well as being skilled, he is looked up to by almost all the other kids. He enjoys getting attention he's never had before, and therefore takes on a leadership position readily. He cares for the younger children, stealing extra food and clothes for them and teaching them thieving techniques as he shows off how developed his are. However, this care doesn't extend to those his age, who he sees at a threat to his position. Sykes is quick to challenge the older children to prove that he's better, and will do anything to win. He usually acts aggressively and even arrogantly to assert his authority and remain the center attention and adoration.
When Link initially declines Ganon's offer, he returns to the hideout. Link asks where Farin is, because he wants to talk to him about Ganon's offer, but Sykes says that Farin went to the prison when Sykes told him about Link's capture. Just then, one of the gang comes into the hideout with terrible news. He was stealing some bread or something and saw guards apprehending Farin and taking him into prison.
Sykes, both because of his habit of putting older kids down and because he actually believes it's true, immediately accuses Link of being the cause of Farin's arrest. Somehow, Link traded his freedom for Farin's. The other children aren't sure, but go along with Sykes. Link feels betrayed and leaves the hideout to find Ganon and accept his offer.
When Sykes hears that Link was asked by the Gerudo King to be his apprentice, this comes as an enormous blow to his psyche. Sykes is a better thief and has done more for the gang than Link has. When Link returns to the gang's hideout after the King's proposal, Sykes is furious and hurt. He convinces himself that Link is a traitor and would give up his ties to the gang in an instant when opportunity presents itself. To "protect the younger kids from Link's traitorous influence" and protect his own position as the coolest kid there, he lies and informs Link that it's Link's fault Faron was thrown into prison. The younger kids, who look up to both Sykes and Link but aren't informed about the real situation, go along with Sykes. Link, feeling betrayed, leaves the hideout to accept the Gerudo King's offer.
When Faron returns to the hideout, he hears from Anton about what happened while he was trying to bargain for Link's release. He has a talk with Sykes, who just feels more angry. Not only did Link get chosen when Sykes should have been, not only did Link turn his back on the gang, but now Sykes is getting in trouble for it!? Sykes, furious, leaves the gang and the city, setting off on a quest to hone his thief skills and become the best thief Hyrule has ever seen, to prove his worth.
One plot twist we could include would be that actually Ganon meant to take Sykes on as his apprentice, but made a mistake and chose Link instead. Just a random, undeveloped idea.
Along Link's quest, Link runs into Sykes on a couple occasions. Sykes heard of the cogs on his travels, and thought that collecting them would prove to everyone that he was the best thief Hyrule has ever seen. Sykes would challenge Link, either to a battle or some other challenge involving thief techniques when they ran into each other.
Something we'd kind of need to work out is that we've also got Maple looking for the cogs. I like the idea of them all searching, but Link kind of has an advantage because he has the thief bracers.
One idea I had was that it'd be really cool if in the third dungeon, Goron Mines, Maple helps Link out during the boss battle. Not as a playable character, but as a mechanism that could be really cool. And perhaps he even gets the cog at the end instead of Link, and runs off with it. Then later in the game, it would be stolen from her, either by Ganon or by Sykes, working for Ganon (But of course we wouldn't know it was Sykes until the end).