Yuki technically doesn’t have a last name, but she’s a sexaroid by designation (a throwback reference to character creator’s earliest work of the same name, and a term for the kind of robotic humanoid she’s a part of.) She’s the medical officer for Sirius Platoon.
Her behavior doesn’t really give away her robotic origins, but for a good part of the first season, she doesn’t really consider herself much like a human, thus avoiding some human mannerisms like socializing, instead choosing to live solely for her duty as the medical officer of the Big 1. However, after having her life saved by Manabu while chasing down a group of wanted criminals, she allows herself to be persuaded to take part in social events.
Yuki’s personality is generally calm and somewhat withdrawn, but not without a certain sense of curiosity. (Especially after the aforementioned incident.) She doesn’t hold her own life much of value, and are prone to making rash decisions in situations of emergency whenever a humanoid life is threatened, dismissing her actions with the reasoning that she “can be replaced if something were to happen.”
Nevertheless, she seems to enjoy her work, and she did once profess that she had a certain affection for Manabu, whom looked like someone she had known in the past—possibly her creator.