、？
Koga, what on earth happened in the first half of your life?
Literal
Koga, you [attributive-の] half-life [at-で] on-earth what [subject-が] existed-[past-の]?
Casual but dramatic inquiry. 半生 ('half a life, the first half of one's life, up to the present') uses 半 ('half') + 生 ('life'). 一体 ('on earth, really') is an intensifier for questions expressing astonishment. The の at the end is the feminine/casual question nominalizer. The speaker seems to be reacting to something revealed about Koga's past.