The Fine Line is Will the Echidna's personal Extreme Gear. It possesses the unique ability to change both color and rider's ability depending on the costume Will wears.
The Fine Line was made alongside the Life Surfer during the two year between books one and two in Flashfire212's fanfics. Made by Kyle the Fox, the Fine Line was designed with a trick up its sleeve. Designed to read Will's emotions, it could switch between a 'light' and 'dark' setting. Kyle worked hard on the board, and was pleased to see Will take to it well. The peace wasn't to last though. Scourge invaded and Kyle was killed fighting Striker, Will's anti-form. The board broke soon after and was taken to Ronan the Fox, a genius and master with mechanics.
At first, Ronan said the board was unfixable on his first look. However, as he took it apart, he saw Kyle's genius in the emotion link to it's rider and decided to optimize it. He used DNA coding to set it so only Will could ride it, then went into more fine detail. A chameleon circuit went in, turning the board into something that fits whatever costume or clothing Will wore. The emotion sensor was next, with Ronan making sure it also detected Chaos Energy present in Will's body, allowing the board to get better stats when he powered up. Finally, he fixed the hover mechanism which had clogged with goo (later revealed to be quick-setting jelly dropped by Knuckles) When Will had it returned, he hopped on the board and road off to the statue of Kyle that had been built in a park at the orders of King Elias Acorn.
It was at this statue that Grinder the Hedgehog found him, telling him of the Eggman in his realm's plot where he had a device that turbocharged Chaos Emeralds when used in the Eclipse Cannon so he only needed three to make it work. Will reluctantly agreed to race Sonic boomhog and enter into his group to stop Eggman.
In Sonic boomhog Extreme Formation, Will has a lot of abilities, most of which are unlocked through alternate costumes on this board.
In Will's normal appearance, the ability is Chaos Flare, one of Will's signature abilities. It causes the rider ahead to lose balance and sometimes fall off their board.
In his Anti-form costume, the ability is Rage Shield, where Will uses his anger to create a shield around himself and his board, blocking everything around him for a 30-second time span.
In the Pilot's uniform, Will's ability is Thruster Boost, where Will pushes a mass of Chaos Energy straight into his board, giving himself a 5-second boost and creating a backdraft behind himself, knocking people around and slowing them down.
Will, as the Legendary Echidna Swordsman has the ability Sword Dash. He holds both his swords out, one on each side of him, then has a 5-second boost, knocking people of their boards and damaging the boards.
When Will uses the power of the Chaos Emeralds, Sol Emeralds or Master Emerald to go Blazing, the Fine Line transforms into the Fire Wall. It has the ability Solar Flare, the more powerful form of Chaos Flare.
Will retains the Fine Line, even into his adulthood and the Rageik saga. After his kids are born, he has Ronan make a second board for his son.
Striker is able to ride the board, though he is Will's Anti-form.