性格の強さ /sei ka ku no tsu yo sa/ would mean 'stregth of personality/character. 性格の固さ /sei ka ku no ka ta sa/ also can be used, meaning 'firmness of character'. If you mean something like discipline it is 規律 /ki ri tsu/ meaning 'order, discipline'.
Yuuki:courage by the way... a Japanese "sign" like this one is called kanji
It is "Chikara", meaning power, energy, spirit, or strength. It is a kanji character.
Strength.
The word power, as in strength, is 'chikara,' written: 力
'Chikara' or 'tsuyosa'.
its tuff
grit, guts
Strength...japanese words written as "力(ちから)", pronounced as [ti ka ra]. Improvement: Close enough, yes, but the usual pronunciation is "chikara".
Character development.
The name "Matsumoto Rangiku" is of Japanese origin. Matsumoto is a common Japanese surname, while Rangiku is a unique given name often used in Japanese manga and anime.
Hello Kitty is a Japanese character because she was produced by the Japanese company Sanrio who continues to market the character to this day.
the character is Chinese (Kanji in japanese), but the pronouciation is Japanese as it had already existed before the character of 犬(inu) entered Japan.
力 (chi ka ra) is general Japanese word for 'strength, might'. But if you mean e.g physical strength then other words are made out of it like 体力 (tai ryo ku) or for dynamic power, motor power 動力 (dou ryo ku).