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.