Loneliness.
Rex knew this feeling all too well. His father died when his mother was seven months pregnant with him. When he was born, she met the same fate and succumbed to her injuries. The doctors were just able to ask for a name for the child:
Rex. Rex Soli Tudo was to be his name.
She turned to her child, smiled with the last of her strength, and said to him:
Make honour to your name and live like a king. That is my only wish.
Rex was then taken to an orphanage and raised there, where he spent the next 18 years of his life. Because he was never taught how to socialise and because of his calm and quiet personality, he was unable to make friends and became increasingly isolated. He spent most of his time reading, and his immense thirst for knowledge became apparent. He read books on history, medicine, foreign cultures, chemistry and so much more. He also always did his best in school and achieved top grades. This "peculiar behaviour" did not please the other children and they isolated him even more. This continued until he was 18....
He then joined the military, where Rex showed talent with firearms and martial arts. However, due to his lack of socialisation and his resulting uncooperative behaviour, Rex was referred to the secret service, where his high intelligence and general knowledge played a major role.
From then on, he was successfully used for espionage in several countries until a planned assassination attempt on a queen failed and he was exposed as the man behind the whole operation...