Chapter 21

Observation (観察, Kansatsu) is the 21st chapter of the Parasyte manga series, written and illustrated by Hitoshi Iwaaki.

Overview
''Hideo learns about Migi's weakness, where he must sleep for multiple hours a day, and informs Reiko of his findings. Shinichi becomes less trusting of Hideo when he must stop him from killing Mitsuo and his friends. Yuko, who has taken an interest in Hideo, learns that he may not be human after witnessing him reshape his face after getting injured.''

Summary
At North High, Yano asks what happened to Mitsuo, who was beat up the day before by Hideo. The group then walks over to find Hideo, who accepts their challenge and leads them to find somewhere quiet. While waiting for Satomi after school, Shinichi is warned by Migi about Hideo's killing intent towards multiple humans.

Hideo finally stops in a quiet, walled-off field, and tells Mitsuo's gang to attack him all together. He can detect Shinichi approaching and wants to kill them before he shows up. However, Shinichi leaps over a three-meter-high wall to get to them. Hideo states that he doesn't want trouble, so Shinichi tells him to leave and he'll handle the fight. Hideo agrees.

The North High students assume Shinichi was protecting his friend, but Shinichi thinks about how he's protecting them. He tells them to go home, then stands in front of Yano, the leader, to make it clear. Yano throws a punch which Shinichi easily catches, squeezing his fist enough to hurt. Yano, realizing that even he wouldn't be able to win against Shinichi, leads the others away.

Hideo gets a call on his house phone and answers, speaking to the former Ryoko. He tells her that Migi seems to be hibernating a few hours each day and they ponder over it. He then asks about Hirokawa and she states he's working with the "five".

At school, Yuko is painting and notices Hideo outside as she's taking a break, calling him Noh-mask boy for his lack of expression. Hideo is currently investigating Migi's signal silence and attempts to wake him up by blasting killing intent at Shinichi, who doesn't react. Yuko then comes up behind Hideo and comments on the surprised expression he wore as well as his general lack of them, saying that it's like his face is fake. She then runs away from Hideo's shock and tells him that if he's mad, he should look mad. Later, Hideo investigates her in turn and realizes that she is perceptive due to being an artist, and that he has to be careful around her.

Haruki Tachikawa returns home while Yuko is watching a show on Mouth-heads, which he scoffs at. He stares down at the sketch of a parasite he'd done during the talk with Kazuyuki, then hides it when Yuko asks about it. He asks her if she knows Shinichi, who she does know vaguely and asks what he's done. Haruki says nothing, just that he met his father at work earlier.

Yuko then brings out a sketch of Hideo and tries to prompt Haruki to see if he notices anything from his experience. He says that he can't get much from just a drawing.

On a later day as school ends, Yuko notices him standing alone and watches the students leaving. A batter hits a baseball that smacks into Hideo's head and conspicuously dents it, shocking Yuko as she realizes his face really is fake.