The reason both Luciano and Blake had suggested that Tessie was the one to be saved was that they knew the capabilities of a blood harvester, especially against beasts.
Apart from the boost in plant control that came with advancing to a higher realm, a blood harvester unlocked 3 abilities that worked hand in hand.
The first was the blood drain. It granted the blood harvester ability to conjure thorny plants/vines that used the drained blood to strengthen themselves, thus saving the user quite a lot of mana.
The second ability was a passive one that granted conjured plants more energy during the daytime. Meaning a blood harvester was more dangerous during this time.
The last was the mother seed and it allowed the user to conjure a seed that would give birth to a thorny tree.