Katakana vs Hiragana


So I'm playing Pokemon Alpha Sapphire in Japanese hiragana (no kanji) to help
with my reading comprehension. I've been noticing some of the characters will
use words that I normally see in hiragana (like きみ) in katakana.

The question is why some of the words are being used in both hiragana and
katakana. So far, most sources I've seen say that katakana is used for names
and foreign words, so I'm confused on why I'll sometimes see the typical
Japanese word in katakana instead of hiragana. Is it common for the katakana
spelling to replace a typical hiragana spelling? Additionally, what exactly is
changing when the typical hiragana word is written in katakana?

