Ser Jonothor Darry was a renowned knight and a legendary member of the Kingsguard of the Mad King Aerys II Targaryen.
In the books
In the A Song of Ice and Fire novels Jonothor Darry's history is the same. Just like most of the other members of the Kingsguard at the time, he too was fiercely loyal to his oaths, fighting for the crown during Robert's Rebellion.
He was present at the aftermath of the Battle of the Bells to rally the remaining royal forces and later on lead the army of the royalists to the Battle of the Trident, alongside Prince Rhaegar Targaryen, Prince Lewyn Martell and Ser Barristan Selmy. He died at the Trident fighting until the end for his cause.
Although Jaime Lannister remembers him as a earnest and honorable man, he (like most of his sworn brothers) did nothing against the atrocities committed by the Mad King, not even intervening when the King raped Queen Rhaella. He stated that they were sworn to protect the royal family, except from the King.