What's the difference between 君 and きみ/キミ ?

[Tyraxx]

Hi everybody,

as the title already says, I'm wondering sometimes words are written in kanji and sometimes in kana. I'm often confronted with this situation in japanese games (visual-novels,etc.).

Now my question is, where is the difference between e.g. 君 and きみ/キミ ?
Is it more formal when it's written in kanji?

Same goes for names, while some call (written) the other person by the "kanji", the other call (written) the person by "katakana"

I hope somebody can give me the answer...