Here is one thing that could be easily overlooked: character can die and can be resurrected. There is even character responsible for it, and I think there should be a way to get his attention or even friendship. So, let's sum up some facts.
1. Characters can be resurrected from death.
2. The parasite can be extracted from dead body.
3. The parasite even leave dead owner, as we can see during the game, and thus can be destroyed outside the body.
I think, that there must be an option to impress the ressurrector, so in certain moment he, after having a small talk with Halsin, suggest to kill you. If you attack immediatly or act agressivly, then this option is closed. If you are calm and listen, then he describe his idea of helping you more specific. Then story is branching:
If you managed not to increase parasite power during the jorney, you get rid of it, otherwise it just loose part of its power and is resistable to this procedure. Also other parasites can't be cured that way, so only one character can be freed.
"They won't repeat the same mistake twice"