A Heart Closed Off
Ai very rarely shows any emotion. She always has the same flat expression on her face no matter what she is doing; observing, giving someone a straw doll, amusing herself or sending someone to Hell. Her eyes are glazed over and you can never tell how she feels or what she is thinking. Ai has been doing her job for over 400 years by the time the series begins, however. She has seen thousands upon thousands of different types of hatred. She's had to send just as many to Hell, regardless of whether they deserved it or not. Being capable of emotion would make her job impossible. Though she does have a strong hatred due to her own past, she obviously does not enjoy her job. She vocally doubts herself quite frequently, and almost acts pained when a request comes through. Her "grandmother" has to tell her a request has come through before she gets up to go many times. Ai would have to be extremely twisted and cruel to be able to ferry thousands of sometimes undeserving people to Hell without a second thought. She is most certainly not an evil person that would be able to experience emotion and do her job at the same time. Therefore, I think it is only natural that she closed her heart off after so many years of such terrible work.Putting the problem with having emotions aside, I also believe a good reason for Ai being so reserved is because she wanted to forget her past. Being betrayed by the only person she trusted and then buried alive with her parents is definitely not something she can look back on easily. And even though she was extremely angry at her village for putting her through such pain, because she broke down sobbing afterwards, I don't think Ai looks back on burning the entire village with any sort of content or satisfaction either. Most of Ai's existence has been filled with pain; the only things that weren't were taken away from her in a painful manner. Since most of the first season builds up to Ai remembering her past, I think it is probably safe to say that she blocked those memories out of her mind. Though Ai is still pretty reserved after remembering, I do believe she begins to show emotion and interact with others more. Therefore, I think that wanting to forget her past was a large part of her not showing any emotion to begin with. The only Ai that she remembered was Enma Ai, the Jigoku Shoujo. What emotion did she have the right to have after causing so many such despair?
After 400 years of watching over hatred, it was probably only natural to lose herself in it and forget who she was. She couldn't escape her punishment, even if she wanted to. For her, there was most likely no point in dwelling over what had already been done. She couldn't change what happened in the past and she had to continue to be the Jigoku Shoujo whether she liked it or not. Even if she were initially furious and falling into despair, without a stimulus (such as realizing Sentarou never died), it ultimately didn't matter anymore. They say that time heals all wounds, and 400 years is a long time to cover up old memories/emotions.
Ai always bears everything by herself. Though she does have companions, she keeps herself at a distance from them. They never actually know what she is thinking or feeling. Wanyuudo tends to be able to sense her emotions as he occasionally makes comments like how he hears her heart grating, but for the most part, she is alone. Unfortunately, I think this puts Ai in a vicious cycle. Because she is almost incapable of expressing emotion, she can't build relationships with others. And because she doesn't rely on other people, she bears more by herself and therefore keeps more to herself. Her companions obviously care about her, but I would guess that Ai probably thinks there is no point in bonding.
The last explanation that I believe could account for Ai's reservedness would be her general apathy towards herself. I discuss this in more detail here, but because she doesn't care about herself, why should she feel anything? She doesn't exist as a person. She has no individuality. She is more or less a doll that only acts when others "allow" her by pulling the string. She doesn't need a personality. She doesn't need a heart. She doesn't need feelings. Her whole existence is simply cause and effect. Someone hates someone enough to send them to Hell, they access Jigoku Tsuushin, she gives them the straw doll, they pull the string, and she sends that person to Hell. Ai doesn't get a break from her work and does nothing to enjoy herself. She believes that she has nothing and is nothing but an envoy of hatred. What is there to feel?
b a c k . c l e a r . f o r w a r d Heaven's a Lie and its contents are © Samantha, however Ai and Jigoku Shoujo are © Aniplex, Studio Deen and other rightful owners. Heaven's a Lie is a part of AFTER-DEATH.ORG.