If you think about it logically, it's obvious that Rhyu Si-min is a spy

At that time, university students who were arrested
while participating in the democracy movement were terrified,
thinking things like 'I'm going to die now'
or 'I have to endure this somehow.'
Because of these complex emotions, their expressions were frozen.
However, there isn't a single trace of worry on Rhyu Si-min's face.
If he were arrested for being detained and assaulted as a civilian, he should feel ashamed,
but he enters with a smile, as if it were staged in advance.