Found: Protagonist travels to other worlds to stop antagonist in each realm

The protagonist was given something I believe from the Uncle that allows the protagonist to travel to different realms to protect them. The book is in a series. Each book is the protagonist is going to a different realm to protect it. I remember distinctly the protagonist losing the first two books. And one of them had a humanoid race I believe that was sort of enslaved and the book did a great job outlining and detailing the humanoids as panther like and smart and athletic. Each time he comes to a new realm the people of that world don't trust him much. If I am not mistaken I believe the panther humanoid is killed by the antagonist at the end of the book.
I read this book/ series just over 15 years ago. The audience for this book is mainly Jr High and High school students level reading.
I remember that the last book in the series basically had the protagonist running between the realms trying to finally win against the protagonist who is a human who also has the abilities to go between the realms just like the protagonist uncle.
Sorry I wish I remembered more.

Pendragon series by D.J. MacHale

Oh my gosh thank you sooo much!!