Izzy is Nadia's grandfather and Ethan's step-grandpa. He's a sweet guy who's gotten a second chance by marrying Margaret. Not only are they totally and embarrassingly in love (says Nadia), but they've moved out of the slow-moving retirement area, Century Village, and into a high-rise.
It's like a do-over. They're starting out on their own, even though they're senior citizens and, as Noah says of people living in Century, should be "retired from useful life" ("Noah".12).
Grandpa Izzy communicates with his granddaughter in a way only she can understand, explaining Margaret's interference by telling Nadia that saving turtles matters: "sometimes one species has to help another get settled" ("Nadia")—whether the ones who need help are people who could use a fresh start, or turtles, still in their eggs, who just need to make it to higher ground.