In manga, katakana is often used for onomatopoeia. In the Dragon Ball manga below, every kind of action is also given a sound effect. In the right page, the kamehameha is given a sound effect represented as キュアアツ. In the left page, jumping is given a sound effect represented as パツ. Truthfully, I think these representations are just as bad as the ones on American comics.
I read that katakana is also used to depict foreign accents or robotic speech in writing which seem to also fall under emphasis. Additionally, katakana (and only katakana) was used in telegraphs years ago. Scienctific terms are generally expressed in katakana.
I found a great website with unusual uses on katakana:
http://gakuranman.com/unusual-katakana-words/
I'll pretend some terms aren't there:
バーター (sidekick) and デマ (rumor) are definitely not loanwords from English, so it could be from another language, but it could also be emphasis. If none apply, these are definitely a special cases.