Lou is a devil type of character playable on Guitar Hero III: Legends of Rock. Lou is unlockable once you beat him in a Guitar Battle at the end of any career on any difficulty. His name could be a reference to Lucifer. You also unlock his Guitar called the "Obliviaxe", which looks like a skeleton corpse (the headstock is a horned skull, the fretboard is a spine, and the body is a rib cage).
Lou plays a great part in the backstory of GH3, as the main antagonist posing as the manager of your band. The band gets hooked up with him after the performance in Mitch's Moose Lounge, where they sign a contract that, unbeknownst to them, gives Lou concession of their souls (this is only revealed after the gig in Kaiju Megadome, when Lou shows his true demonic self).
In the Co-op story, Lou plays a lesser role, only appearing after the band members are arrested because of the fire they inadvertently started in their Backyard Bash comeback performance(the encore was Cities on Fire with Rock and Roll). Lou makes a deal to set the band free as long as they perform for the prison inmates. Doing so, as soon as they step out of Shanker's Island, Lou drags them down to his domains.
Lou, in his human form, also appears in the GH3 tutorial, antagonizing God of Rock.
Biography
Lou's his name and gathering souls in exchange for musical prowess is his game. Meet him at the crossroads and leave your nerves on the porch or prepare to spend eternity tuning his guitars. Definitely a dog person. -- Guitar Hero III: Legends of Rock in-game bio