What makes Hitomi so endearing is that she is a completely normal character. That isn't to say she's boring; far from it. It's just that she's so believable. She doesn't have incredible martial arts skills, beauty, or bust. ^_^ Like any normal person she has flaws: traits that some people find annoying, and other traits that make her likeable.
One of the first things you notice about Hitomi is that she is very caring and friendly. She has a nice friendship with Yukari: they laugh and joke together; Yukari cares a lot about Hitomi as is evidenced by her urging of Hitomi to confess her crush to Amano. Hitomi would not have earned such care if she was not a good friend.
In the first episode, when Hitomi sees a first (of many) visions involving Van's death, she races to warn him, heedless of her own life. One of the neat things I thought when I first saw the series was how much Hitomi saves Van. She doesn't rely on some manly man to save him; she does what needs to be done. Yet Hitomi can also become unsure of herself and confused over her emotions, like any normal teenaged girl. Who hasn't, once in their lives, questioned if they really love someone? Add to the pot powers she doesn't understand that gives her visions of death, and you wonder why she doesn't have a nervous breakdown at some point in the series.
The reason is that Hitomi is a strong character. When she first lands on Gaea, she's scared, yes, (who wouldn't be?) but she doesn't hold a pity party about it. Instead, she has faith in Van and the people he serves, that they will help her find a way home. Which brings us to another aspect of Hitomi's character: faith.
During her stay at Gaea, Hitomi meets up with many characters, some nice, some odd and some just plain scary. Yet, when Hitomi befriends them, she also believes in them, believes that even if they have chosen the unwise path, they are good people. A wonderful example of this is a scene during Episode 23 (Storm Premonition) when Hitomi is cooling down after running and Folken comes up to her. He asks why she isn't wary around him like everyone else in Austuria; her especially, he did try to kidnap her. She responds by saying that "If I was wary around you, you would put up your guard and wouldn't be the person I know you are inside." Even though Folken was a "bad guy" she believed he had good intentions at heart.
For all her good traits though, Hitomi is merely human. As I've already mentioned, she becomes unsure of herself when it comes to her emotions towards Allen and Van; this causes Hitomi to act selfishly and do things that causes some trouble later down the road. (For a more in-depth look at her love life, please go to Heartstrings). Yet almost immediately after she makes these mistakes, she seeks to correct them. During the aftermath of Millerna and Dryden's wedding ceremony, she gives herself up to Zaibach in order to save her friends and undo her selfishness. She takes responsibility for her actions immediately and does what needs to be done in order to help her friends.
In closing, Hitomi's honest portrayal of an average teenage girl, her quick thinking and maturity make her a truly unique and loveable character.