Abstergo has some data on this guy, gathered a few years back. But there's not a lot about him in standard history books, which I'm sure he'd be happy to know. Yusuf Tazim was a Turk born in Bursa around 1467, but was raised in Constantinople from the age of eight by his mother. His father was an Assassin, and presumably that's how Yusuf became one too. But there's no mention of what happened to papa Tazim. Was he killed? Did he desert the family? Your guess is as good as mine.
By the time he was 17, Yusuf was well known around the city as a rabble rouser and a petty thief. But a chance meeting with the legendary Vizier Ishak Pasha changed all that. Ishak was an Assassin too, operating in the Sublime Porte right under Bayezid II's nose. It was Ishak who had defeated Vlad Tepes, and later brokered the tentative truce between the Ottomans and Assassins. By the late 1480s he was busy recruiting open-minded citizens of the Empire to his Order's cause. Yusuf was taken in with Ishak's charisma and easy manner of leading, and so joined the Assassins just before his 20th birthday.
From there, Yusuf's rise was swift and assured and by 1500 CE he had made quite a name for himself. In 1502, Yusuf met with a contingent of Venetian Assassins in Greece in a joint effort to steer the Ottoman-Venetian war to a peaceful end. These efforts appear to have paid off, for in 1503 the fighting stopped, ushering in a brief era of peace on the Ottoman Empire's western border.
Perhaps made complacent by the Assassins' successes in keeping the peace, Sultan Bayezid II grew soft in his final decade, preferring his mystical texts and alchemical experiments to governing. This suited the Assassins just fine, for a dull peace was preferable to endless war and Imperial expansion. But, following the earthquake in 1509, a new threat appeared in Constantinople - Templars fighting under the old Byzantine crest. Stirring from years of relative calm, Yusuf once again roused his Assassins to begin the long and difficult fight ahead.