- "Once, your Creed was as vital to me as air and water... but when the Turks marched into Wallachia and you Assassins did nothing to stop it, how could I continue to believe?"
- ―Vali explaining his betrayal of the Assassins.[src]
Having served the Assassin Order for around a decade, Vali left the Assassins after they made a truce with the Ottomans; the same people who had humiliated his people, conquered Wallachia, and murdered his secret idol Vlad Tepes. Feeling betrayed, he joined the Templars, striving for vengeance against the Assassins and Ottomans.
Born as a noble in Wallachia, Vali joined the Assassins in his early twenties. He remained loyal to the Creed for almost a decade, until the Assassins called a truce with the Ottoman Sultan, Bayezid II.
Believing that truce was traitorous to his people, since the Ottomans had shamed them, conquered Wallachia and even murdered his role model Vlad Tepes, Vali left the Order to join the Templars and seek revenge on not only on the Ottomans, but also the Assassins. Due to his Assassin training, Vali was soon deemed to be one of their most valued and feared killers.
Feud with the Assassins
- Ezio: "Only a guilty man runs with such speed!"
- Vali: "And only a fool stops to fight!"
- —Ezio chasing after Vali.[src]
Being an experienced agent for the Templars, Vali had taken the lives of many Assassins. However, an Assassin apprentice caught on to his actions and spoke to the visiting Mentor, Ezio Auditore da Firenze, who had journeyed to Constantinople.
Arriving at a small courtyard and using his Eagle Sense, Ezio discovered evidence of Vali's recent murders. Suddenly, Vali appeared, running along the rooftops and assassinating an Ottoman guard. The former Assassin then fled the area, with Ezio and the apprentice quickly giving chase.
Later, as Ezio's apprentice attempted to air assassinate him and failed, Vali called out for help, spurring some Byzantine guards to aid him, which provided the distraction that allowed him to escape.
- "Mentor, forget me. You have fallen for a ruse. He captured me to draw the Assassins away from our headquarters. He will be on his way there now!"
- ―An Assassin explaining Vali's plan.[src]
Vali later managed to create a diversion, by capturing two Assassins and containing them under Byzantine guard, in order to give him the time to try and infiltrate the main headquarters of the Assassins, which succeeded in distracting Ezio from his true ambition. After Ezio had freed the captives, Vali ordered Byzantine forces to attack the Galata den, with the fight ending with heavy losses on both sides.
Soon afterwards, Vali and Ezio had a brief encounter, which resulted in his attempt to escape. However, he was soon stopped in his tracks by the same Assassin apprentice as before, who emerged from the rooftops and leapt down to assassinate him, this time proving successful. Before he passed, Vali explained his actions and why he had abandoned the Order, claiming that he could not believe in a cause that he felt had betrayed his people. Once he had said his final words to the two Assassins, he gave his last breath.
Behind the scenes
Vali is a Romanian diminutive of names that ultimately derive from the Latin valere ('to be strong') or valens ('strong, potent, vigorous'). The latter portion of his name, cel Tradat, translates to 'the betrayed' in Romanian. Internal files of Assassin's Creed: Revelations name him Vali cel Umbra instead, with cel Umbra meaning 'the Shadow' in Romanian.  This may indicate that cel Tradat acts as an epithet, not a surname.
A playtester at Ubisoft Bucharest at the time of development was the one who came up with the name and concept of the character and urged Québec to include him in the game at the request of his uncle, who is also a Romanian. 
Ironically, Vali's secret role model Vlad Tepes is a member of the Templar Order, which could mean that his defection was not solely driven by a conflict between loyalty to the Assassins and Wallachian nationalism—admiration for a Templar ruler is thrown into the mix.
Even if Ezio manages to assassinate Vali before the apprentice, the scene in the Memory Corridor still shows the apprentice giving Vali his last rites, reflecting the canonical killer.