The Quintessential Quintuplets: Ghost/monster: オバケ vs おばけ but I don't see the emphasis

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Why are katakana preferred over hiragana or kanji sometimes?

Popular: モテ vs 人気

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In the anime adaptation of the manga The Quintessential Quintuplets, as well as the corresponding manga, I notice there are 2 times 'obake' was used. I finished the anime s1 and s2, but I'm just now reading the manga, and I notice there's a difference in obake used in 2 scenes - 1 is with hiragana and 1 is with katakana.

Scene 1: S01E01

Re 2 of the quintuplets Nino (who is older, if relevant) and female protagonist Itsuki: Nino says to (or about) Itsuki 'この肉まんおばけ.' (You can see 0:06 - 0:10 here.)

Scene 2: S01E08

Re the siblings Fuutarou and Raiha, Raiha (who is younger, if relevant) asks their dad if Fuutarou, the male protagonist, was not always a...

Anime Subtitles: ... a 'study freak'.

Manga English: ... a 'study monster'.

In the anime and manga original, Raiha asks '前はこんな勉強オバケじゃなかったの?' (You can see 1:57 - 2:02 here.)

I was curious but never really bothered to look up the exact furigana/hiragana/katakana or whatever of what Raiha said which sounded like 'benkyou baka (ばか)' (like...study idiot?). But when I saw 'study monster' in the manga. I got really curious and then saw - ok the Japanese word for 'monster' here is indeed the same as 'monster' for Itsuki. So it was actually 'benkyou o(something)' and then 'bake' instead of 'baka'.

Now comparing both:

So it appears both protagonists here are described by their siblings as monsters...but the difference is the katakana vs hiragana:

1. Katakana オバケ - is used by Raiha for Fuutarou.

2. Hiragana おばけ - is used by Nino for Itsuki.

Based on the above linked questions, I guess it's to do with bold/italics or something. But then...I'd think it would be the reverse. At least in the anime, Nino says obake with emphasis while Raiha doesn't. It's like

1. For Nino: (You can see 0:06 - 0:10 here that there is emphasis on 'obake'.)

Nino says: kono nikuman OBAKE!!!

2. For Raiha: (You can see 1:57 - 2:02 here. I really don't think there's an emphasis on 'obake here.)

Raiha says: Mae wa kon'na benkyō obake janakatta no? (That's it. There's no bold/italics on the 'obake' here.)

Question: What's going on? Is Raiha's actually emphasised while Nino's isn't?

Answer 1

Nothing special is going on. Don't try to overanalyze it. This is a word that are commonly written in both ways, and most people do not have any opinion about which to use. Perhaps the author adopted whichever IME showed first without thinking anything. I can hardly imagine the author had some concrete intention for this, but even if he did, it would be impossible for anyone other than the author himself to explain it.

If you insist, hiragana tends to look slightly feminine, cuter or childish, but I don't think that works as a strong reason here.