Protection and revenge are indeed instincts within the human heart. Ian also understands why, during the Spirit Disaster, he chose these two inclinations as the foundation for Spirit Energy inheritance.
Moreover, unlike the progress of the Protector Knight which relies on 'Miles.'
The Spirit Energy related to the Avenger Messenger can be entirely drawn from the various Spirit Energy abilities of the Deceased Monarch—there are many indeed, as most of the Deceased Monarch's abilities are driven by the anger and unwillingness of the Lost Souls.
In other words, the Avenger Messenger is the 'Deceased Monarch' who uses their own power to drive relevant abilities.
No, without subjects, they cannot be considered a 'Monarch,' only a messenger of their own desire for revenge.
If there were no other unexpected events, Ian would certainly choose the 'Avenger Messenger' as the next development direction for the Spirit Energy inheritance of the Supreme Spirit Sect.
