I'm not sure specifically what 'Cap deage' means. It could be a very specific term, a misspelling, or something relatively unknown. It might be a proper name, a local term in a certain area or a very niche concept.
Webtoon does have a few 18+ comics. However, their availability can vary depending on your region and the platform's content regulations. Some might be hidden behind age verification or specific access settings.
I don't have enough information to say exactly who Cap'n Kathy is. She could be an original character created by the fan fiction writer. Maybe she's a pirate - the 'cap'n' part could suggest that - who has some connection to the wizarding world of Harry Potter.
No, webtoon and manhwa are not exactly the same. Webtoons can come from various countries and have diverse styles, while manhwa specifically refers to Korean comics.