After watching episode 19:
Bu Yeon is the daughter of the Jin clan and the Choi clan.
- Her father's extreme inferiority complex made him think Bu Yeon is not his daughter but Jang Kang's.
- Jin Moo knew she is his real daughter.
- Ironic, for a father who's thinking Bu Yeon is not his biological daughter (and hated her for it), he reacted like any father would when Jin Moo pushed her to the lake. Deep inside, he knew she is his daughter.
- Bu Yeon's affinity to the ice stone came from the Jin side.
- Bu Yeon's ability to override the Alchemy of Souls came from the Choi side.
- If Bu Yeon is in the vicinity when an Alchemy of Souls is casted, Bu Yeon can do whatever she wants with the spell.
"To save someone you love sometimes you have to let go of everything."
- Then the scene shifted to Nak Su.
- She has to let go of everything. Of her past. Of her pain. Everything she knew as Nak Su. To save the person she loves, Jang Uk.
"It is up to you to choose not to use that power."
- Fitting to "let go of everything". (see above)
- Especially now that Nak Su is in Bu Yeon's body … she has access to the power of the Jin clan, as well as, the Choi clan. Not to mention her own as Nak Su. She's one scary mage right now.
- Bu Yeon's plan was well orchestrated too. She made sure Nak Su will change and choose to protect the person she loves.
New AoS rules revealed in ep19:
- If the two souls are conscious, even if on different bodies, it makes them weaker.
- The blue mark can be easily removed.
- Even if they return to their own body, they remain a soul shifter that who will eventually turn into stone and ash if they don't replenish their energy. - Mu Deok will eventually petrify if she depletes her energy.