In Parasyte: The Grey, Heidi, who is named by Kong-woo, is the parasite that is inside Su-in’s body, but Heidi doesn’t function like other parasites do.
Heidi enters Su-in’s body when Su-in is dying. She has been stabbed multiple times by a rude customer who argued with her at the supermarket where she works.
The attacker comes to finish his job but is immediately cut in half by Heidi. Once recovered, Su-in senses something is wrong with her and learns about the parasite inside her body from other parasites that follow her.
Heidi voluntarily communicates with Su-in. It is revealed that Heidi is unlike other parasites out there. She is a mutant, and her goals don’t align with those of other parasites.
The reason Heidi is different
Heidi, much like other parasites, followed her instinct as a larva to take over a human brain. Su-in was nearby, and she entered her body.
Since Su-in was already dying, Heidi was not able to completely parasitize Su-in. Heidi tried to synchronize with the entire body and was forced to heal Su-in first.
She succeeded in doing so but failed to take control of the brain. Now that Heidi doesn’t have complete control over Su-in’s body, she can only come out when Su-in is in danger.
Heidi has done her research on Su-in’s body and has come to realize that she can only take over for 15 minutes a day.
Hence, Heidi has to preserve her energy and only come out when required. Su-in’s body maintains Heidi’s life, and Heidi has to protect it.
Heidi’s fate
Till the end, Heidi works with Su-in and Kang-woo to stop the other parasites. Residing inside Su-in helps Heidi understand the positive side of humans and emotions such as trust.
After defeating the leader of parasites from Saejin Church, Su-in gets back to her supermarket job. Kang-woo, who is now part of Team Grey, visits her and hands her a letter from Heidi.
In the letter, Heidi mentions that she is still thinking about the meaning of their existence, but she says it was a great stroke of luck that Su-in is the human she parasitized, and she was unable to take full control of her brain.
Heidi seems to have left Su-in’s body to her, but it is suggested that she continues to reside in her. The last words of Heidi say that Su-in is not alone, whether she likes it or not.