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Yaba has both abilities. He can heal by singing and destroy/harm by screaming.
Spoilers ahead!!!
The best way the novel put it is the Yama is a siren. He can heal but he can also harm someone.
In the novel there is a scene where Yaba heals the ML (forgot his name) while having sex.
But as you said when healing the ML brother his health is deteriorating, but when Coca heals him, it actually gets better.
Do u rmb the doctors who operates on them? Castrate them into eunuchs? The one who killed them is actually Yaba. lol
Later there are 3 types of healer, Yaba is a siren which means he can even bring someone back to alive. With sacrifice of course lol (((srry that's too much spoiler)))
I didn't read the novel but i don't think as i see from what we know so far they both have the ability to heal or destroy its more up to what they want. coca's main mindset is to heal, he's kindhearted and wants to help people. yaba isn't evil or anything but his mindset is much less focused on healing people and so far he only truly used his powers strongly when he wanted to hurt someone. also, the meds yaba had been forced to take probably weakened his ability or something in my opinion, they definitely have something to do with how messed up he is right now.
So are coca and yaba supposed to be opposites? Coca heals and Yaba destroys because in all the incidents where Yaba sings there seems to be either death, or it seems like the man who he’s singing for (forgot his name sorry) health seems to be deteriorating