Caesar is the Graystone family's dog. He is the puppy of Daniel Graystone's previous dog, and grew up with Zoe Graystone, outliving her mortal self after the Maglev 23 bombing. In the aftermath, Caesar bears witness to Lacy Rand's revocation of security clearance after Daniel discovers her in Zoe's room, and lead him to discover Zoe's avatar in the V-Club (CAP: "Pilot"). Caesar has access to Daniel's lab and exhibits familiarity with the U-87 robot, prompting Graystone to suspect that Zoe's avatar still exists (CAP: "Rebirth" and "The Imperfections of Memory").
In an attempt to convince the U-87 to reveal itself as Zoe, Daniel gives the robot a handgun and orders it to shoot Caesar. It complies after determining that the gun is loaded with blanks due to the weight differential, sparing the dog; otherwise, she might have turned it on Daniel (CAP: "Ghosts in the Machine").
- As stated in Serge's twitter account