It is unlikely that Greninja will rejoin Ash in the anime.
Greninja's departure in the Kalos region marked a significant moment in the series. While Ash's journey in the Alola region focused on new Pokemon, Greninja's absence was a constant reminder of his past adventures.
Here's why it's unlikely Greninja will return:
- Greninja's Departure Was Emotional: Ash and Greninja shared a powerful bond, and their farewell was emotional and poignant. Returning Greninja would diminish the impact of that departure.
- New Pokemon, New Adventures: Ash's journey continues with new Pokemon and new challenges. It's essential for him to focus on his current team and the relationships he develops with them.
- Greninja's Role in the Kalos Region: Greninja's role in the Kalos region was significant, and his departure marked the end of that chapter in Ash's journey.
While fans might hope for a reunion, it's important to remember that the anime focuses on Ash's growth and development, and bringing back old Pokemon might hinder that process.
