I know, I wrote that I can see Ruri being the guide - she'd definitely do it to cause trouble for Sou, ideally to the point where his family forcefully pulls him out of school. But there are things that make this unlikely. First of all, Ruri belongs to another class and more importantly, she's a guard team leader. The schedule for "Class Observation" states that assigned guards will do their usual job. As such, Ruri has to escort Mairu and cannot play guide for another male student's family - that would set a bad example for an outside observer. My bet is on Yuri since she's the Vol. 4 poster girl... as to how that actually comes to be, who knows. I can think of several options, like:
- Yuri volunteers herself. The class will probably object... would she do that? Her splits might make her to
- Yuri gets pressured into accepting it by her classmates - though I doubt this will go undisputed
- the job automatically goes to the fourth ranked student which might be Yuri - but is she a high ranker? I doubt it.
- the class is fighting for the right while only Yuri appears indifferent - the teacher picks her because of that
- Sou has a word in this and suggests Yuri because she's his most trusted confidant ... hard to refuse in that case
- the class draws lots for the position and Yuri wins despite not even wanting to participate
Of course kisa-sensei will pick and even more hilarious option.