Otona is a native Japanese word meaning 'adult'. In Chinese characters, it's normally written meaning 'large person'. The Chinese characters for otona are a rather special case. While conveying the meaning of the word, the characters are quite divorced from the pronunciation. They have, in effect, been arbitrarily assigned to write otona because the Chinese language, whence the characters came, does not have a single word or character that means 'adult'.
This is the normal usage in Japanese, as in the following advertising sign in a department store:
It can also be found in hiragana as :
In special circumstances it can also be found written in katakana as , as in the poster below:
This is a poster for a summer-holiday movie directed at the juvenile market. Class 3A under the Japanese system is composed of 15-year-olds declaring war on 'adultdom'.
By using otona in katakana, the poster achieves the following effects:
The fact that this is a minority usage is demonstrated by the results of a Google search of the Internet in August 2003, which showed that the Chinese character version is by far the predominant way of writing otona: