The idea of a person carrying an unchanging character through their whole life is something that only happens in novels.
Literal
Certain person [subject-が] unchanging character [with-で] whole-life [object-を] pierce-through [nominalizing-ということ][topic-は] novel [genitive-の] inside only [genitive-の] story is.
A literary/philosophical observation. ある人物 ('a certain person,' 'some individual') uses ある before a noun as the indefinite article — 'some X' or 'a particular X whose identity isn't specified.' 全人生を貫く ('to run through all of life,' 'to traverse one's whole life') uses 貫く figuratively — literally 'to pierce through,' here 'to carry consistently from start to finish.' The ~ということ nominalization treats the entire proposition as a concept. The predicate 小説の中だけの話 ('a story only inside novels') is a cutting dismissal — this kind of thing is fiction.