Though he is said to initially have a gentle and sweet disposition, Isaac became sad when Sandeman left. He howled and cried loudly for his "Father" to return, but the guards at Manticore became fed up with his noise and began to punish him for refusing to stop crying. To keep him quiet, they cut out Isaac's tongue, leaving him traumatized by the event and terrified at the sight of men in uniforms. As a result, he became feral and violent.
Isaac appears similar to Joshua, but has shorter, darker hair and a smaller build. Although Joshua and Isaac resemble each other greatly, they are not twins. Isaac, unlike Joshua, has a genetically-encoded barcode on his neck.
After Manticore's Destruction Edit
After the destruction of Manticore Isaac began attacking men in sector police attire, the sight of them making him relive the moment he was abused and believing they were going to hurt him he ripped out their tongues and killed them. He has a brief encounter with Logan Cale and Alec McDowell in the sewers when they go looking for him but locks them both in his hideout with all his collected tongues. Max Guevara eventually finds Isaac whilst searching for Joshua and ends up fighting him. As he is about to kill her Joshua steps in and stabs Isaac causing his death. Joshua then buries Isaac outside of Sandeman's house.