[Nyororin]

Japanese names are generally written in Kanji.

Jack is not a Japanese name - there is no correct way to write it in kanji. Go with katakana, it is the only correct way to write it in Japanese.

I am going to guess that Marika is your Japanese friend`s name? If so, I would be VERY careful about writing it in kanji without direct confirmation from your friend.

There are countless ways to write the same name in kanji. Think of it along the lines of spelling, but with far more variety. It would be incredibly embarrassing to go to all the trouble and then find that you have written her name with the wrong kanji... if it is written in kanji at all. Occasionally Japanese names are written in hiragana or katakana.