I think the word genki describes Sana perfectly personality-wise. As I have stated on the main page, genki means energy, vitality, vim, stamina, spirit, courage, pep, and a variety of other things. Sana has all of these qualities and more.
Sana comes across as an extremely hyperactive young girl. She talks a mile a minute, often times so fast that you can't even understand what she's saying. I'd have to say that Sana-chan is *the* most hyper anime character I've ever seen. Her energy seems almost limitless at times. She just keeps going and going, running around, arms a flailing. She's a complete spaz.
Sana is also a very optimistic, kind hearted and caring person. She sees the world as a wonderful place full of opportunities, and as a result, can be a bit naive at times. Even so, she's a genuinely caring girl. Whenever she sees someone with a problem, she does her best to help that person in any way she can. For example, she takes Rei into her home when he is homeless, and she's determined to help Hayama when she finds out about his family situation.
Sana always means well; her good intentions are genuine. However, she can be a bit on the oblivious side at times, especially when it comes to others' feelings. After her "relationship" with Rei came to an end, she became a bit weary of relationships in general. She's completely unaware to Tsuyoshi's feelings when he's in "love" with her. And well, there is of course the situation with Hayama. Sana is oblivious to his feelings for her for a *very* long time.
At times, Sana uses her happy exterior as a front to hide her true feelings. If she's upset, she just smiles and laughs as if nothing is wrong. She finds it easier to deal with her feelings this way. If she cries, then something is definitely wrong. Hayama seems to be the only one who can sense when things are wrong with Sana. He is also the only one she'll cry around.
Hayama: "I think you know, you're good at acting. That's why, sometimes I think that your cheerfulness is an act.. and not always natural."
Sana: "But you know, people tell me it's not like Sana-chan if I'm not energetic, so I'm always happy and hyper!"