The Sims 3
Since his parents, Jonathan and Caridad, have yet to meet in The Sims 3, he nor his siblings are born yet.
The Sims
Neither Zachary or his twin appear in The Sims. But it appears their conception in imminent since Elise is already a child.
The Sims 2
Zachary begins in The Sims 2 living with his sister Cordelia and her two children, Percy and Mariah in Pleasantview. He and his sister are revealed to be the children of Jonathan and Caridad Ionello, as well as the siblings of Elise Whelan. Zachary was close with both of his parents, and like his two sisters, was devastated by the loss of his father. He also has a good relationship with his extended family, often seen with his cousins Jordan Ionello and Damien Pola. He loves his nephews and niece dearly, and has a knack for children. He gets closer to them much more easily than adults.
Zachary is frequently flirted with by Kaylynn Langerak, and also has a good relationship with family-friend Daniel Pleasant. He has thick black hair, expressive eyes, and is not a stranger to the ladies. He wears a green sweater accompanied by blue jeans. He's moderately far into the Natural Scientist career, and has a level 7 in logic. As his bio implies, he lacks a life of his own. He shares his life with his sister by helping with her children, and working for her benefit. It's apparent that his mother encourages him to do things for himself, but sees no issue in changing what he's doing now.