The main character in this story, Key is indeed a strange girl. Honestly being a robot despite the fact that no one believes her, her mannerisms are that of an autistic child. She is not a robot so much in the fact that she does not feel human emotions, which she certainly does (albeit usually very repressed), as much as in the fact that she just has no idea what is going on in the world. Her "robotic" personality is based more on innocence rather than the stereotypical heartless machine. I personally can definitely relate to her, as she represents someone who just simply has no idea what the heck is going on and is just trying to find someone that will relate to her.
I'm not saying she is entirely perfect though, because she does occasionally crack up at the end as her human emotions become progressively more intense. I suppose that was the ironic moral of the story, maybe robots really are saner than humans.
Not much is mentioned about him as a person, and at first glance he seems to truly care about his granddaughter. However, one must keep in mind before assuming he's a "good guy" that he did do many things that are hardly what one expects of a do humane individual, such as using both his wife and daughter as labrats, even continuing after despite the pain they went through, only stopping after he accidentally killed his daughter. Still, he has at least shown guilt for it and had the conciense to keep it from Ajo (who will be discussed later) and even eventually scrap the project. I think part of the reason he told Key to become human is not so much that she would perish if not doing so, but because of guilt due to having to keep her as a robot for her entire life. Either way, as much as he may have accidentally hurt people, he did feel guilt for it.
Dr. Wakagi is a rather mysterious individual indeed, and if there was one charachter that I wish I could have seen more about, it was him. It would have been good to see more of how he ended up getting involved with Dr. Mima (although this could have been simply that it was a job he worked at at first), and more about the apparantly deep relationship between him and Sergei. However, I suppose I can forgive alot of that, due to the fact that this is a sort of enigmatic individual that one is hesitant about coming too close to. For so much of his past to really be covered, that would probably make him come across as less mysterious, now wouldn't it?
One thing that many may notice is that yes, the only thing he seems to care about is Key. Did multiple people just get killed and injured that Key could probably cure? Take her home and leave them that way. Sakura got killed? Oh well, life goes on. She shouldn't have been associated with Key. That's what some people are thinking, right? Well, in his defence for the first one, it was obvious that to cure one person was a very stressful situation for her and she was coughing up blood and fainting at the end. If you were Key's bodyguard, would you want to see what would happen to her if she kept doing that multiple times, and with someone who thought she had infinite power pressuring her into doing it? Probably not. Besides, he has clearly given up any sort of every day pleasure to live in Tokyo to watch over Key, at once shown going to bed at a park bench. If I had to give up all that just to watch over something, I'd be pretty uncaring about what I had to do to keep it safe, too.
I have many mixed opinions of Sakura. True, she is perhaps the one person other than Dr. Wakagi or maybe Tataki that has helped Key the most, and you have to appreciate someone that can live through the hell she has. However, she, too, has her own set of personal issues. She seems to suffer from some form of bipolar disorder, one moment in a perfectly happy and friendly mood, then set off from one tiny event. She has on multiple times proven to be very rash, and has made harsh comments to Key that really, you just don't say to someone that impressionable. Nevertheless, I do for the most part like her, because despite the difficulty trying to raise Key may be, and the less than pleasant lot in life that she has been handed, she still manages to try to put on a pleasant face and go on with life while at the same time selflessly trying to help a friend when no one told her to.
Like Sakura, Tataki is another relatively normal person whose world is brought upside down by Key's presence. It seems to me that beforehand, he was living in sort of his own bubble of Miho fandom and chatting with Sakura, a bubble that unfortunately pops, just not to the tragic extent that it does for Sakura. It is clear from the beginning that he is a very laid back and generally happy person, the sort that you would not expect to be in such a dark series, and I am pretty sure that is the way he has always been beforehand. This is why overall, I feel that he is the one charachter in the story who changes the most. The stress from everything that happens, starting specifically during the time Key and Tamari fall from the building, seems to turn him into a far more paranoid person. Even after this first event, he is still shook by the intense situation but is still able to joke lightheartedly with Sakura about Key's supposed delusion. The time he goes to Key and Sakura's hometown is a major turning event to me, where afterwards the images and events he saw there has driven him to a state of paranoia. Plus, dealing with the occasional callousness of Wakagi, the underhanded tactics of Prince Snake-eye in the effort to keep Key out of showbusiness, and the bizarre informational shutoff of Miho don't help to assauge his nerves.
Before anyone judges him for not noticing Sakura's feelings for him later on, I can't really blame him. Even excluding the fact that the guy can't help that he has his own obsessions, how was he supposed to read Sakura's mind? She never specifically said that she had any feelings for him, and at times will shut him off. It's just the way he is.
Ajo is perhaps one of the most irredeemably evil and insane villains in any anime I've seen, and this is coming from someone who always sort of roots for the bad guy. For example, anyone remember Gendo from Evangelion? I sort of like him, I sort of feel bad that he lost his wife and I think that he does in some distant way care some for his son. Ajo on the other hand just never has any sort of soft moment, and as far as we know was never before a better person (in a flashback story, far back during WWII Dr. Mima met Ajo and even then thought he was dangerous). He has a fetish for robots that is just disturbing, to the point that being touched by a human being makes him physically repulsed. Of course, there's the fact that he even tells his technicians to more or less choose a sacrifice out of the three for a "gel" extraction. All in all, I like to tell myself that I'm lucky to just not work for this guy.
I like Miho alot, and she is most definitely my favorite character in the entire series. Really, I think in a lot of ways she is more tragic than Sakura, which is ironic since you wouldn't think a pop artist would be that way. I am aware that she is one of the characters that suffers the most character bashing in the minute Key fan base there is, and I felt the need to comment on it. Many people claim that she got what she deserved, and was rather weak to let such a thing happen to her. I won't argue that it was entirely out of her grasps, but how was she supposed to know beforehand that this tool which is supposed to drastically improve her performance when her hope of being a professional musician was probably down the tubes, and probably from someone that completely neglects to tell her the drawbacks of it. I would accept it too, and yes, I probably would keep using it even if I started to feel worse as a result until I'm in a condition and position (contract possibly) where it was too late to turn back too. Given the way she was forced into privacy so much, I wouldn't be surprised if right when she was offered the chance to use the machines that she had to force a contract more or less saying she would continue working for him under any circumstances.
I could go on for a long time about her, so I perhaps should just make an entire page dedicated to her and just go on.