Diagon

Diagon is the one of the four main antagonits: the other three is Shredder, Kraang and Dormammu and the leader of the Flame Keeper's Circle.

Hamato Kyle's younger days.
When Hamato Kyle [or just Kyle for short.] was yound and was found out that he has the same power as the Esoterica the Esoterica then brought Kyle to him but due to Diagon bonding with him, he became his adoptive father to him, and told him that the humans would't apriceated him becuse of his powers.

First attempted and trapped
On the day Shredder attacked Yoshi the Flame Keeper's circle used a device [also known as the Time Dilation Accelerator]  that could travel to other dimensions but was stopped by the other two dimension turtles and was temporliry trapped in Diagon's dimension.

Out and went missing
After realizing that there was still some power in the accelerator Kyle used it to get out Diagon's dimension but realized that he have splitted up from the group so until he was found and regaind his strength Kyle was the current leader of the Flame Keeper's Circle.

Found in New York
When moving to new york one of the Esoterica found Diagon in a circus and reported to Kyle and they later resced him.

Trivia

 * Hamato Kyle is the only person he cares about [like Shredder with Karai]