One of the more bizarre facets of Rise's storytelling was the reveal of resonance, which refers to two distinct phenomena.
First, a monster's ability to influence and overpower the emotional states of other creatures. This is foremost expressed by Ibushi and Narwa, exerting some kind of influence that terrifies lesser monsters and drives them out of their territory, resulting in panicked hordes that trample most anything in their path known as Rampages. The Apex monsters have a reduced capacity for this influence, allowing them to enrage any hordes they are part of.
(Magnamalo is immune to the effects of the Serpent's resonance, allowing it to freely exploit the Rampage for easy meals. In biology, this is a form of symbiosis called commensalism; one species directly benefits from the actions of another, who itself is neither harmed nor blessed by the symbiont)
Second, a Wyverian's reception to the emotional states and even motivations of other Wyverians and monsters. For Hinoa and Minoto, this is initially their attunement to each other's emotional state, allowing them to note how the other is feeling and better synchronising their speech and motions. It also makes them oddly sensitive to a monster's resonance; they can feel the rage and terror of an Apex, and most distinctly they are so overcome by the emotions of Narwa and Ibushi they seem to lose their minds and start speaking with the thoughts of the Serpents.
Now, this isn't actual mind control or the Serpents 'possessing' them. The emotional resonance of such powerful monsters is so strong that Hinoa and Minoto are overwhelmed by the radiance. They are essentially 'tuning in' while the Serpents commune with each other, like a twisted take on eavesdropping. This is initially debilitating to the twins, but they are able to steel themselves and endure the strain of resonance to track the location of the Serpents, leading the Guild to the Coral Palace.
So after all that, this leads to an inevitable realisation on just how awful the implications of resonance can be. Even far in the Coral Palace, Serpents can provoke emotional reactions in Hinoa and Minoto, notably telling when Ibushi is distraught or Narwa is furious, for example. Just what is the extent of resonance's influence then? When Ibushi and Narwa are subsequently hunted down and slain by Kamura's Fierce Flame, did Hinoa and Minoto feel the pain and fear of the Serpents during their final moments?
If resonance is so potent even the simple instincts of these creatures can force such reaction, I can't imagine Hinoa and Minoto were in any happy state when the Serpents were in their death throes. It certainly adds a darker edge to when you make the twins Followers and bring them on quests to slay Ibushi or Narwa. Not only is one directly attuned to a particular Serpent and thus privvy to its suffering, the other sister will be feeling that anguish through the twin's own bond with each other.
Small wonder Capcom didn't reference it in Sunbreak.
