Why when being romantically affectionate do these ex-romantic partner step-siblings call each other by last name? (Motokano S01E01)


1st episode of new anime adaptation of light novel series My Stepmom's
Daughter Is My Ex aka Mamahaha no Tsurego ga Motokano datta:

* * *

Yume Ayai and Mizuto Irido have recently become step-siblings. Before they
were step-siblings they used to date. It seems they broke up at least 1 year
before becoming step-siblings.

In the very 1st scene, we see that Yume calls Mizuto as Mizuko-kun and Mizuto
calls Yume as
Yume-[san](https://japanese.stackexchange.com/questions/95318/how-relevant-is-
gender-in-comparing-last-name-kun-chan-compare-to-1st-name-san).

> [![enter image description
> here](https://i.stack.imgur.com/P1P6b.jpg)](https://i.stack.imgur.com/P1P6b.jpg)

> [![enter image description
> here](https://i.stack.imgur.com/1thtc.jpg)](https://i.stack.imgur.com/1thtc.jpg)

Later, they play [Kaguya-
sama](https://anime.meta.stackexchange.com/questions/4509/is-it-ok-to-
overanalyze-an-anime/4513#4513)-like games with each other to try to trick the
other into not acting as normal siblings. They manage to trick each other at
least once each. Now, also once each, when 1 of them gets tricked, they call
each other by last name: Irido-kun, Ayai.

> [![enter image description
> here](https://i.stack.imgur.com/YPXba.jpg)](https://i.stack.imgur.com/YPXba.jpg)

> [![enter image description
> here](https://i.stack.imgur.com/WPtfU.png)](https://i.stack.imgur.com/WPtfU.png)

(Ignoring the issue of the presence/absence/choice of honorific...I'll just
pretend for now it's the same as 'Himuro (no honorific)' and 'Yukimura-kun' in
[Science Fell](https://japanese.stackexchange.com/questions/94180/translation-
of-rikei-ga-koi-ni-ochita-no-de-sh%C5%8Dmei-shite-mita-is-science-fell)...)

**Question** : What's the idea here? Is it like, Idk, 'simulating' when they
1st started to have a crush on each other and thus weren't so familiar with
each other and so as part of the simulation they would call each other how
they initially called each other then?




I know nothing about the anime nor the manga, but according to
[this](https://originalnews.nico/222881), it is something like

  * First, they call each other by first names, just like siblings. San/kun suggests that they behave like they just know each other.
  * Yume gets annoyed by being called by first name, and proposes a game where one loses if s/he behaves in the "non-sibling-like" manner.
  * Mizuto takes the strategy of becoming "serious", then Yume is kind of taken aback, which led to calling him Irido-kun
  * Mizuto points out Yume has lost.

About the changing to "Irido-kun", it does mean Yume was stripped off her
"looking down/mocking" attitude, more or less in line with the suggested
interpretation. About the change to "Ayai", you can understand it in the same
way, but calling her by "Ayai" seems original to Anime.




I know nothing about the anime nor the manga, but according to
[this](https://originalnews.nico/222881), it is something like

  * First, they call each other by first names, just like siblings. San/kun suggests that they behave like they just know each other.
  * Yume gets annoyed by being called by first name, and proposes a game where one loses if s/he behaves in the "non-sibling-like" manner.
  * Mizuto takes the strategy of becoming "serious", then Yume is kind of taken aback, which led to calling him Irido-kun
  * Mizuto points out Yume has lost.

About the changing to "Irido-kun", it does mean Yume was stripped off her
"looking down/mocking" attitude, more or less in line with the suggested
interpretation. About the change to "Ayai", you can understand it in the same
way, but calling her by "Ayai" seems original to Anime.

