Spoiler alert
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From what I know, she somehow received the novel they're supposedly in and learns from it the struggles that awaits her. She did not want to go on the expedition and do all the hard work of defeating the demon lord but she did want the glory associated with the title of a guardian. So, the father-daughter pair used black magic to bring mc into the body just for period of the expedition and switch it back right after. However, the switch back failed for some reason and that's why the mc was in a coma for 3 months. And, og!selonia was supposed go through character development during the expedition. Since she never struggled during that period, she remained selfish and spoiled.
Base on the latest chapter, it might have something to do with his father's evil plan. And since she's a good daughter who has magic power, she always follow his every commands, swap body, use dark magic and summon a different soul as a sacrifice or something. She didn't know the soul she summoned was put in her body and that it will change her every character when she uses dark magic. I already knew her father was a sus.

I just don't get something, why did the real Selonia changed bodies in the first place? Like she now wants hers back, so why or what was the reason of the first change? ( ̄∇ ̄") That's the only thing I just can get my self to understand about the plot, or it's explained later on? Can someone explain it to me?