Silver Heart

Silver Heart (Formerly known as Agent Silver Heart) is tone of he main three Phantom Thieves teachers and the adoptive "Grandfather"/ caretaker of Queen.

Before Series
Like Joker, Spade and Queen, he lost his parents at a young age. He was then enrolled in a Spy academy and become a spy, in service of a queen of a kingdom.

Saving a Kingdom
50 years before the series, Agent Clover to destroy the country by using the forbidden weapon hidden in the mountains, known as the Pandora that contained one of the 3 crystals. Sliver succeeded in stopping him, but injured his left eye in the process, with Clover losing his hand.

Becoming a Phantom Thief and the 3 Apprentices
He quit becoming a spy and became a legendary Phantom Thief. He saved Diamond Queen when she was a baby under a request of her parents in the burning castle in his heist for a treasure.

At one point in time, he tries to steal the Time Telling Goddesses from the Jones mansion, but young Joker  was still present in the mansion. Later when Clover and Doubt attempted to kill Joker and get the treasure for their plans, Sliver saved him by deciphering Joker's parents hidden message of the key and fighting Clover and Doubt. Joker then joined him after he witnessed the miracles of a Phantom Thief. King was later taken in after King joined Joker and Queen during a heist on the ship he was in. Roko, a talking super dog was then trained under him.

Trivia

 * His three apprentices have one thing in common; they lost their parents at a young age under the hands of Professor Clover as they have the things needed for his plan.
 * He prefers Queen over all apprentices