Shia is a very gentle person: always polite, calm, supportive of others and cheering them on. She says things that come from her heart, and it's hard to not like her. People feel like they can trust her and confide in her. However, she can be quite oblivious as well; imagine you're confessing to someone, and only receive a "thank you" as a response. And with this, I might have portrayed Shia like a person without any troubles in life. However, people are not always what they seem to be, and Shia is no exception to that, because remember, Shia is a demon after all, someone who is usually displayed as an evil form of life. Although Shia is a person who many people are fond of, and we often hear Nyaa scolding Shia for being incompetent, Shia does have the ability to be evil, be it unknowingly. In the anime, it's limited to the headaches Kotarou and Misha get in the earlier episodes, but the manga has a couple more examples, like Shia sucking out Kotarou's life force, or biting him when he's asleep. At the end of the manga, it is even said that perhaps Shia was a true demon after all.
Although Shia looks to be a young, shy and innocent person, she has actually gone through a lot of (hard) situations. She always believed that she was the source of problems in her family, and when she had formed a family of her own, she has had to leave them behind in order to protect them and forgetting all about them. And when coming back to the human world, she found out that everything she sought for was already gone. All these things made her, in my opinion, able to be mature about things and to better understand people and comfort them.