Upsilon, the Greek symbol for U, is an A.I. unit that belongs to Agent Florida. Upsilon is one of the more advanced A.I. units, as it was developed as a "Delta 2.0," meaning a better version of Delta. Upsilon was involved during The UNSC Freelancer War. Not only did he help Florida out in his missions, but he was originally an A.I. unit for the UNSC, until being stolen by Agent Florida and Slash.
In the begining of their adventures, Upsilon, or at the time Delta 2.0, hardly was noticed as he was following Florida's orders to, keep quiet. He was then later thought to be corrupt while they were trying to get to Florida, as North Carolina does not lead straight to Georgia. It was later found out that Upsilon was not corrupt, but that is how he was programmed as South Carolina had already been destroyed like Florida. Upsilon was then accidently fused with Gamma during the battle with O'Mally and Gamma, which caused Upsilon to become corrupt, forcing Florida to eject Upsilon before it did any more damage.
During the middile section of the adventures, Upsilon was corrupt, so he was not included in these parts. However, before being ejected from Florida, Upsilon had left traces of himself in Florida's mind, but so had Gamma. Gamma was too powerful for Upsilon while Florida was unconcious, but while he was concious Gamma could not control him, leaving Upsilon in control.
At the end of Chapter 4, Upsilon had been defused from Florida and was returned to him as well. Florida inserted Upsilon into his helmet, who was still known as Delta 2.0, and then told Upsilon that he learned about what his new name was supposed to be, Upsilon.