The Laughing Lion.

AU: Tywin and Joanna Lannister had a son in between their twins and Tyrion, his name is Jason Lannister named after his maternal grandfather.
As a young child, Jason swore to his mother to protect his newborn baby brother Tyrion from any harm as any older brother should for their little brother.
Leading him to become cold hearted and viscous in the process from his father and sister's treatment of Tyrion.
Once Tyrion is arrested for Joffrey's death, Jason's gloves will come off and plans to make those who condemn his brother for a crime he didn't commit to suffer a fate worse than death.