Mo also passed basic training alongside Kerry Chang, Gabrielle Connor and Shakeel in "The Recruit" and was described as extremely bony. He was apparently found in a box at Heathrow airport at the age of four- his parents have never been found.
He wasn't seen again until The Fall in which he attended James' birthday party (although his name was on a list of James's phone numbers at the end of The Killing). His disappearance has never been explained.
Mo is a notably minor character and doesn't even have a profile on the CHERUB website. He is also the only agent not to be given a surname.
During a flashback in Shadow wave when James, Kerry, Kyle and a mix of older and younger agents including the twins and Mo himself. Mo winds James up about Arsenal losing to Manchester United and his stirring eventually leads to James being kicked out