One of the main themes is deception. Lady Audley has many secrets and she uses deception to hide them from others. Another theme could be the position of women in society at that time. Lady Audley's actions are, in a way, a reaction to the limited options available to women. Also, the idea of appearance vs. reality is prominent as her outward charm hides her dark secrets.
Lady Audley is obviously one of the main characters. She is the one with the big secret. Then there's Robert Audley, who plays a crucial role in uncovering her secrets. Also, George Talboys, whose relationship with Lady Audley is an important part of the story.
I'm not entirely sure as I haven't read it specifically. But generally, a light novel might be about the story of a 'Secret Lady', perhaps her adventures, her hidden identity, or her relationships in a fictional world.
Love is likely to be a main theme. Often in such novels, the relationships between characters, especially the romantic ones, are central. Another could be secrecy itself, as the title implies that there are secrets held by the lady in the story.
Maybe it's a story where a lady gets divorced and rebuilds her life, finding new hobbies and self-confidence. It could involve her traveling and having adventures to discover herself.
Well, in many such stories, the so - called secret might be some kind of magic potion or a special ritual. But without knowing the specific 'the secret to eternal life story', it's hard to say for sure.
Perhaps it implies the untold tales within the journey of life. Every individual has a unique life journey, and there are parts of it that are like a secret story, such as the small moments of self - discovery, the behind - the - scenes of relationships, and the internal growth that others may not be aware of.