Isaac is a character in Mother 3.
Isaac and his mother lived alone in a small house on a hill. He is the one who foreshadows Hinawa's death - the first warning sign is him seeing Hinawa while he is out and about in the mountains, collecting mushrooms. He then hears Mecha-Drago's roars, which Hinawa was trying to ward off to protect Lucas and Claus. He then hears some heart-rending screams as Mecha-Drago stabs Hinawa in the heart. He also tells Flint that he should go back to the Twins' House to check on things and ensure Hinawa and the kids arrived home. Due to the fact that he hadn't witnessed first-hand the Mecha-Drago's attack on Hinawa, he cannot tell Flint exactly what happened.
In Chapter 8, Isaac can be found outside New Pork City's theater talking to Butch. He is now a Pigmask, and expresses surprise that Lucas is the important guest that Porky Minch is looking for, saying that he is a big shot now. Isaac also warns that he and Lucas will be enemies should Lucas do anything to defy Master Porky.
- In EarthBound 64, Isaac was called Yosaku, and was going to be a caveman-like body builder with mostly the same role.