I've read that ヒロイン , meaning 'heroine', can also imply a romantic interest to the hero pretty much by default culturally in Japan's stories.
I can't seem to find any definitive source for this from a native speaker, though, so I'm wondering if anyone on the language speaking boards has heard this before?
In a love story like Romeo and Juliet, yes, ヒロイン refers to a main female character who will (eventually) fall in love with a main male character.
In other types of stories, ヒロイン can simply refer to the most significant female character in a story, and you can use ヒロイン when there is no romance concerned at all. A ヒロイン tends to be young and beautiful, but can be an old lady, too. For example Joan of Arc, Yotsuba Koiwai, Pretty Cures and Maiden in Black can be called ヒロイン. Still, 主人公 may be a safer term when there is no romance concerned.
ヒロイン has a few definitions. In most cases, it means the female protagonist in a story. It can also mean a heroic female character. Another meaning would be the female partner of the main character. ヒロイン can also take on the meaning of a 'love interest'.
See this Wikipedia entry.
"pretty much by default culturally"は言い過ぎちゃうかな・・みたいな？