Siri's friend Pag. In childhood Pag gets bullied often. Siri has lost most understanding of social norms and emotions due to surgery, but he still had a mind to get the bullies off him by using rocks on them until they run. Siri still thinks of Pag as a friend even in the present.
Siri's realization that Isaac Spindzel's knowledge of biology and cybernetic implants meant he could essentially No Sell Siri's Awesomeness by Analysis abilities and make Siri see whatever he would have chosen to make him see — something Cunningham makes full use of — but chose not to and let Siri read him. Siri realizes that Isaac, despite referring to him as the "commissar", was the closest thing he had to a friend aboard.