Vivian had also accepted to join, she understood well that she couldn't free her father from his torment unless she grew stronger. She had to master her new abilities and she had to reach Tier 6 at the minimum.
Only then could she hope to be able to release him, though her chances would still be low. Her dad had been Tier 7 in life after all. If he had fallen to the temptation of undeath he would be far too strong for her to contend with. This however she did not believe would be the case for even a second.
She suspected that he was fighting daily to remain human, to not let go of his sanity and his memories. That his soul was being tormented daily. The thought made each day hell for her, but it also strengthened her resolve.