1st episode: Why does Raiha call Itsuki by 1st name (presumably without knowing the existence of the other quints until S01E04)?

Question

What I understand is that the quints are called by their 1st names, in situations where they would normally be called by their last names, for obvious reasons (that they're quints. Extend in general to half/step siblings of the same age an last name).

In the 1st episode, we see that 9-year old Raiha calls 16 or 17 year old Itsuki as Itsuki-san (or Itsuki nee-chan in the manga) instead of Nakano-san (or Nakano nee-chan). But it seems Raiha doesn't know about the other 4 quints until S01E04. (See below.) So what's going on? Guesses:

1. Itsuki's just being friendly to Raiha.

2. Or Itsuki in fact does tell Raiha about the 4 quints, but Raiha forgets about the other quints when seeing what simply looked like 4 doppelgängers.

3. Or maybe out of obvious lifelong habit Itsuki just tells people to use 1st name.

Edit: I'm actually not so familiar with all these honorifics and stuff. What I'm asking really is like

1. Do we really expect that Raiha to call Itsuki as Nakano-san but then for some reason Raiha doesn't?

2. If no, then why? If yes, then why the change?

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See here:

Anime:

Manga:

Answer 1

I'm not very familiar with the manga/anime, but assuming Itsuki is a friend of Raiha's brother, I guess a realistic option is Itsuki-san/chan or Nakano-san. So the answer to Q1 is 'not really'.

The choice of 'Isuki-san' sounds natural enough mostly because it is acceptable for a girl (Raiha) to treat someone as a real sister/brother. Using 'Nakano-san' would sound polite and more distancing/less friendly. So basically using Itsuki-san etc. means that Raiha is generally an outgoing (not shy) type and being friendly to Itsuki.

[Edit] The above means, regardless of whether Itsuki has siblings, there is nothing strange about Raiha using Itsuki-san when calling her. But of course, if there are more than one characters with the same last name, it would be more likely that first names are used. So from the creator's point of view, he(?) chose Itsuki-san so that Raiha didn't have to change the way to call Itsuki later.

A crude explanation is that using first names is rude, but it is allowed (more) for children. For example, if a 25-year old man meets a partner of his 30-year old brother, and talks to her by first name-san, it may sound too friendly and last name-san would be more likely.

Another famous example of this is Conan the detective calling Ran Mouri as Ran nee-chan.

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Even among Japanese people, how to call sisters/brothers is not strictly fixed, let alone brother's friends.

1. 友達の姉、弟のこと、なんて呼びますか？友達に言う場合と直接呼ぶ場合どちらも教えてください。
2. 「兄・姉のことなんて呼ぶ？」4000人に聞いてみた
3. 名前呼び

1 is about friends' siblings (No1 says: when directly talking to brother/sister (of a friend), I would use 「○○（名前）さん（君）」, where 名前 should mean first name.); 2 is about real siblings; 3 is about calling-by-name in manga. But note this is talking generally and not about rules.