Shion is a kind and gentle, but generally naive and idealistic person. He has an amazing IQ level, proven by his acceptance into the Special Course and ability to treat and stitch up the bullet wound in Nezumi's arm at age twelve. When he was restrained to his own bed by Nezumi, rendered motionless, he seemed fascinated rather than scared of his acquaintance's ability.
Despite his nature, if someone or something he loves is in danger, he doesn't hesitate to save them. He is not easily angered, however, he is quick to burst out in violent ways. This is seen when he becomes defensive of Nezumi, attacking and hitting Rikiga when the latter insults Nezumi. In the cave, when Nezumi struggles against Sasori, Shion snaps and wraps wire around Sasori to strangle him, only stopped after Nezumi forces him to. He has also shot and killed a guard, who had in turn gravely injured Nezumi. Shion seems to take on a different personality when he does so. This is proven when he murdered the guard, becoming shocked and frightened once he had pulled the trigger.
This darker nature is only hinted at in the anime and manga, but in the novels, it is addressed more often. Nezumi himself openly admits that he fears Shion, and Inukashi picks up on this as well. In the underground cave, Sasori calls Shion a "demon".
Though grateful for No. 6, he seems to have a slight doubt about it and the whole system, as noted by Nezumi. He is insecure about many things including his slender body and loves his mother very much. He also seems to like to daydream, but is capable of being aware of what happens in his surroundings when he does. [Source: wikia]