Sung is a doctor working for Abstergo Industries. She was responsible for caring for Daniel Cross' physical and mental well-being after he returned to Abstergo in 2000, and functioned as his doctor and psychiatrist until his death in 2012.
Meeting Daniel Cross
In 2000, Sung was tasked with extracting information from Daniel Cross, a Templar sleeper agent who had previously climbed the ranks of the Assassin Order. With Daniel bound to an operation bed, Sung had to inject him with a serum and ask him for information related to the various Assassin hideouts he had visited over the past two years, which led to a Templar operation named the "Great Purge".
While a Templar team was infiltrating an Assassin safehouse in Philadelphia, Stearns, the Abstergo executive overseeing the operation, ordered Sung to inject Daniel with a higher dosage. Though Sung heavily protested, citing the risk that it would have on Daniel's physical state, she relented and followed Steams' order when the latter pointed out that she could easily be exterminated.
Later, in 2002, Sung monitored Daniel's Animus sessions, and would often warn him of the dangers of overexposure to the Animus. On one occasion, she caught him in the Animi room, reliving memories even though the Animus being used wasn't even turned on. She explained the Bleeding Effect to Daniel, who said he wanted to relive the memories of his ancestors because they were the closest thing he had to a family. She reassured him that Doctor Warren Vidic and the rest of Abstergo all considered him to be family.
Caring for Daniel
A decade later on 23 September 2012 Sung called Daniel after hearing he had refused to go on an assignment in Denver, following his part in an incident there. Daniel was glad to hear from her and apologized for missing their last session and promised to make it to the next one. Sung asked how he was feeling, and he reassured her that he was better than he had been thanks to her help after the Denver incident. She then tried to convince him to take the assignment in Denver, but Daniel became angry and cut her off, swearing that he would never return to the city.
On 13 October, Daniel telephoned Sung in a state of great uneasiness, to express his concern following the news of Project Siren's collapse and the death of Lucy Stillman. Sung was worried about hearing about a classified project, and attempted to get him to calm down. Despite his panic, Daniel eventually agreed to meet with her in person at Rittenhouse Square to share his worries. After getting him back to the Templars Philadelphia base, Sung put Daniel into the Animus for several days and increased his medication to help his mental state.
Daniel later suffered another mental breakdown while on a mission in Italy, killing several Templar operatives during an engagement. Sung began treating him once more, putting him back in the Animus for more sessions and confining him to his quarters. On 15 November, Sung called Vidic and reported to him that Daniel had escaped the Philadelphia facility after hacking into the security system and "neutralizing" the guards outside of his room. She attempted to take full responsibility for his escape, but Vidic cut her off and asked her not to inform anyone else of Daniel's escape before hanging up.
After Daniel was returned to the Philadelphia facility by Otso Berg following a confrontation with Desmond Miles, Sung once again began treating him, although she came under pressure from Vidic to clear Daniel for duty. On 24 November, Sung received a call from Vidic, who ordered her to clear Daniel for duty over her medical protests, and informed her that he would be taking responsibility for Daniel's care from then on.