So maybe I've found out who the mysterious Yamamoto Emiko (who's rumoured to be Nokoru's fiance) is...
Duklyon boys' math teacher is a Shoten character named Taana Kurishuna Vishuvaanara Yamamoto (Krishna).
Now I checked in my Shoten Separate Volume 1 his profile and he had a little sister whose name is made by two kanji. I can't read/find the first but the second is definitely a 'ko' (子). So I went to check my Shoten 1-4 (sadly I still miss 5-6) and I found out that his little sister showed up in 'Shoten 3'.
I can't get exactly what the story is about...
we see Rudora Shiva Mahaaviira (Shiva), Utako's preschool teacher, under the rain, Krishna sees him and gave him his umbrella then... end up home with fever to be scolded by his little sister... ^_^; (who by the way looks like a copy of Utako).
Later we see Shiva talking with Nokoru. On the wall there are the portrays of a man and a woman, whom, I've the feeling, they assume are dead.
I think Shiva got by Nokuru the job to avenge them?
Then we see Krishna's sister with her father (who looks like a copy of Ashura-Ou... only with a Taishakuten's expression). Shiva's hidden in a corner of the house with a katana and, apparently, attack him. We see blood spraying.
Later Shiva enters in Krishna's room. He's the one who got cut! Krishna helps him and apparently, Shiva remains with him for the night.
Morning. Krishna wakes up alone and he's awaken by his little sister telling him... someone attented their dad's life?
Krishna and family go to Nokoru to ask for explanation.
Shiva's there too. Nokoru rambles about the two guys that were in the pictures (can't get a word, sorry).
Then takes hands with Shiva's sister as Shiva and Krishna look at the scene holding each other close and... well it seems like Shiva's father his giving his blessing at Nokoru and Shiva's sister.
Later we see a church with two very small figures in front of it that might be Nokoru and Shiva's sister (she might be in a bride dress... or just an elegant one... too small picture to get it)... Shiva and Krishna watch the scene in very elegant clothes.
The resemblance between Utako and Shiva's sister made me worry that the little child I saw in the earlier Shoten volumes was her as well and not Utako so I went to check.
It's Utako. The surname's written the same, the name has different kanji but the last one can be read as 'ko' and Krishna calls her 'Uta-chan' so it's her.
Now... 'Shoten 3' is a cinema dedicated volume. So... hum... it's possible what we see isn't real Shiva's family but just another 'action movie' Nokoru forced then to do as he would later do involving Kentaro, Takeshi and Akira in a 'Duklyon vs. 20 Menso'... (Shiva's sister, as far as I could see was never called by name... however in the first scene she showed up in a box there's written something plus 'Okawa Utako'. It can be: 'She's Okawa Utako' or 'Okawa Utako has this role').
Anyway... that's the story.