Og character is acused for killing the crown princess, she couldn't hold the grief and attempteed suicide
(that's the FL was hanged on the tree)
the og character did a ritual before suicide (the summoning circle)and our FL possessed 
the body when she died , our FL was a bright person in og world and had sun like personality (can relate it to sunshine girl theory) u can say bcz the FL is from other world and is transgimitted to book world she posees an overwhelming aura around her , also the ppl of this book world also worship the sun god (sunshine girl) it gives our FL and god like aura , that's why she appears scary to others , and the ppl who she helps , they look at her as a god and starts worshipping her