When she was here in America, she was known as Utada, but that's weird, because she's now known for her family name. In America, we usually use family names last, but in Japan, they recite the family name first (sorta says something about their culture, now, doesn't it?)

But, yeah, you would formally call her Hikaru like you would call a man by the name of John Smith, John. In Japan, John Smith would be known as Smith John because the family name comes first.

It's confusing, but that's why she's called Hikki. It's because, when she was growing up in New York, no one could really say her first name, which was Hikaru, so she was known for the nickname Hikki (before she knew what a hicky was )

And yes, there are only two formal Kingdom Hearts games (Chain of Memories was just a bridge between the two) and she sang the theme song for each:

Simple and Clean: Kingdom Hearts 1
Sanctuary: Kingdom Hearts 2

And, as most of us know, most things start out in Japan, so she created alternate Japanese lyrics for both of the songs with essentially the same melody. The Japanese songs are called:

Hikari: Kingdom Hearts 1
Passion: Kingdom Hearts 2

The only difference is the language (and the translation isn't the same in both). Check out the lyrics and the songs and translation.

Hope I help