Bleach TYBW part 1 finale revealed a shocking post-credits scene as Ishida Uryu was seen betraying Ichigo and his friends by joining Yhwach and the Sternritter. While the same was earlier hinted through several anime-original scenes and the opening sequence, fans didn't expect the character to join Yhwach himself. Thus, the anime has set him up to have joined the enemy right before the end of part 1.
Through a few anime-original scenes, Bleach TYBW has shown fans how Ishida Uryu came to know about the Father of the Quincy, Yhwach, and the Sternritter. Even episode 12 revealed how Ryuken Ishida discovered the notes Uryu had been reading all this time. As shown in the anime, the latter had read all about Yhwach and the Holy Selection, leading to him joining the Quincy.
Disclaimer: This article contains spoilers from the Bleach manga.
Bleach TYBW: Why did Ishida Uryu betray his friends and join Yhwach?
While it may seem like Ishida Uryu turned his back on his friends at the end of Bleach TYBW part 1 finale, he never actually betrayed them.
While he did join Yhwach and the Sternritter and became the Emperor's designated successor, it was a ploy to infiltrate the Wandenreich Empire and act as a double agent for the Soul Society.
After Ishida Uryu researched Yhwach and the history of the Quincy through the book he found his father's study, he learned Yhwach's role in the death of his mother, Kanae Katagiri through the Auswählen or Holy Selection.
When Yhwach started regaining his powers nine years ago, he conducted the Holy Selection, through which he took the powers of those Quincys he considered impure and made them his own. Due to that, Uryu's mother, who was already weak, passed away.
Thus, the character wanted to avenge his mother, and for that, he decided to infiltrate the Wandenreich Empire and possibly get close enough to Yhwach to kill him. This is why when the Sternritter Grandmaster Jugram Haschwalth came to fetch Uryu, he willingly went with him.
The second reason why Uryu decided to become a double agent was that he was the only protagonist who could infiltrate the Wandenreich Empire, due to him being a Quincy. Thus, he planned on helping his friends from the inside.
Even after joining the Sternritter, Uryu would do the bare minimum in helping them, as even his clash set to take place with Ichigo will be just for show.
Why did Yhwach choose Ishida Uryu as his designated successor?
After Ishida Uryu joined Yhwach and the Sternritter, the Father of the Quincy revealed to his subordinates at a gathering that he had chosen Uryu to be his designated successor.
This news surprised the Sternritter and Uryu, and he felt the need to ask the reason behind the same. Yhwach revealed how he considered Uryu to be special, given how he was the only "impure" Quincy to have survived his Auswählen or Holy Selection nine years ago.
While the Wandenreich king himself did not know the reason behind the same, he believed that Uryu must have a great power hidden within him, that could one day surpass his own.