From Bulbapedia, the community-driven Pokémon encyclopedia.
Isaiah (Japanese: イザキ Izaki) is a character of the day who appeared in Clamperl of Wisdom. His English voice actor is Matthew Nichols and his Japanese voice actor is 小西克幸 Katsuyuki Konishi.
While living on Maisie Island, Isaiah found a blue pearl, similar to those made by Clamperl. He wanted to do some experiments on it so he could discover more about the object. However, when Team Rocket overheard him talking about the pearl, they sneaked in and stole it.
Along with Ash, his friends and Spoink, Isaiah found the pearl inside of a Clamperl, which believed it was its own. Once again, Team Rocket intervened and stole the Clamperl. But the kids didn't give up, and were able to rescue Clamperl from Team Rocket.
Spoink was able to convince the Clamperl to give the blue pearl back to Isaiah. Now with his pearl safely returned, Isaiah began his research on it while the kids continued their adventures in Hoenn.