This girl is one of the main characters of the hit manga series Dengeki Daisy... one of my very favorites.
We don't know really a whole lot about Teru's past, beyond the point of her brother's death. Her parents are gone, and her beloved brother, and only remaining member of her family also passed away several years later for unknown reasons. From information that we've gathered, we assume that Kurosaki had some sort of hand in this, but the details are vague at best. For some reason, Kurosaki (known to Teru as Daisy) has been assigned, by Teru's brother, to be an indirect guardian to her.
Teru is given a phone with only Daisy's number in it. He's been her constant pen pal, or ghost writer, ever since. Unknown to her, the series begins with a situation where Teru is potentially in need of a little assistance in regard to bullies. Daisy has taken a job as a janitor at her school in order to watch over her more carefully. Of course, he takes this opportunity to lend a hand. Perhaps out of character for Daisy, Kurosaki actually confronts Teru after she breaks a school window later on, and she in turn has to work off her debt under his direction.
The odd thing about Teru is that, even though Daisy is supposed to be her outlet and emotional support, she hardly ever tells him just how bad things are. Perhaps this is exactly why Kurosaki decided to step into her life in person.
This being said, perhaps she is a bit naive, but then she isn't. She's naive in the way that she believes in people a lot more than she should. She ends up in situations that are potentially deadly simply to try and help a friend. But she's sharp too. When she's learned something, she understands it and applies it to her outlook on the world. She understood instantly that someone was trying to get ahold of her cellphone to get at some technology that her brother supposedly had hidden. She doesn't care about being direct in some situations, but in others she'll beat around the bush. With Daisy/Kurosaki, she may out right lie to him to protect the feelings that she has for him.
She's tough in that she hardly ever complains, and yet weak when the world decides to crumble on her. During those times, she somehow is drawn to Kurosaki, even before she realizes that he is Daisy. Even with his rough exterior, she is somehow able to tell that he means her no harm.
Teru as a character is probably one of the most complex by far. She feels a wide range of emotions but sometimes chooses whether or not to express them. She's extremely intelligent. Even before she had total proof that Kurosaki was Daisy, she had suspicions that he was. Even her ability to connect with him on a different level than she did with everyone else, even when he was determined to push her away meant that she recognized him as a friend, even before she knew any better. There are things that still sting her. Her world is still largely full of innocence, but she also recognizes that as her world grows darker, the one person who can probably save her is Kurosaki... the man who's suffering the most. But while she understands many things, Kurosaki is always one step ahead of her. Just where that road will lead them though, remains to be seen.