After Robert Baratheon's troops entered King's Landing, Balian refused to abandon Aerys. He remained in the throne room after the rebels had already taken the city. After Eddard Stark entered the throne room he found a dead Aerys and an insane Balian. Balian was imprisoned in the Black Cells following that day.
Balian was born in Highgarden to Lord Luthor and Lady Olenna Tyrell. In his youth, Balian was considered to be weak and a fool. However, Olenna described Balian as the strongest Tyrell who ever lived, until his fall into insanity. Balian was offered to become the Master of Laws to King Aerys II Targaryen, which he gladly accepted.
During his service as a small council member, Balian was the only person the Mad King fully trusted. He was also the only person aside from the Mad King who liked Hersham Groatworth, who also sat on the small council as the Master of Coin. Balian was entrusted with more serious cases which Aerys didn't trust to fall on the other council members, including the death of Brandon and Rickard Stark. At the start of Robert's Rebellion, Balian began to lock himself away from society and subsequently shunned his own family.
Balian was responsible for the death of several noblemen who sided with Robert Baratheon, which, at the end of Robert's Rebellion, was one of the main reasons why he was imprisoned. During the Sack of King's Landing, Aerys demanded for Tywin Lannister's head and ordered Jaime Lannister to kill him. Jaime instead killed Aerys by stabbing him in the back. Balian, infuriated, attacked Jaime but was tackled and knocked out cold.
During Robert's Rebellion, Balian was described as "chubby", being slightly overweight. Like his brother Mace, Balian had curly brown hair, but with a balding top. He was clean-shaven and had a smooth, spotless skin. He had strong hands with short fingers and shining white teeth. He wore clean formal clothing accompanied with a green cloak with golden embroidered roses, the sigil of House Tyrell.
After his imprisonment and subsequent fall into insanity, Balian looks nothing like how he did before. He became dirty and unwashed, his well-styled brown hair had grown white and extremely long, covering his head entirely. He is now an emaciated old man due to complete isolation. He constantly looks as if he is visited by ghosts daily, possessing the infamous "thousand-yard stare". His teeth have become yellow and sharp, like fangs, in order for him to consume the rats that occupy his cell more easily. He wears the same dirty old prison rags that were given to him when he was first imprisoned. He still has his green cloak but it has now become stained with feces after years of using it as toilet paper.
Balian's voice is hoarse and smoky, in contrast to his soft and high voice he possessed prior to his fall to insanity.