Justin was an imaginary playmate of Uriel Septim VII; young Uriel even insisted to set a place for her at every meal. Despite the boyish name, Justin was a girl.
King Symmachus kept up the joke long after she went the way of imaginary childhood friends. He asked about Justin's health every time he met Uriel and the Emperor always replied in as mock-serious a fashion. The Emperor, according to Barenziah, once joked elaborately to Symmachus that she met an adventurous though incorrigible Khajiit youth, married him, and settled down in Lilandril to raise fire fern and mugworts.
By asking about Justin, Barenziah made sure that Jagar Tharn imposed the Emperor.
She is mentioned in The Real Barenziah, Book V.