Don't you mean the fifth? I still think that Tsugumi has something to do with the keys and promise, doesn't remember much from before claude found her.
I can see somethimg like this happening:
Claude: Seichirou, I can't believe it's been 10 (actuallu 11) years since I found you.
Tsugumi: Yeah, because of that I was able to befriend ojou.
Claude: Yes, you've come a long way in these years, not long ago you were that little boy who didn't even know his own name and now look at you, you have been able to protect ojou by yourself all this time, I'm proud of my student 'but you still won't join me in rhe bath, it would be good to bond as men'.
Tsugumi: I'm honored that you think so. 'Not gonna happen' (since I'm a girl, realize it already).
Claude: ahh, now that I remember I have been holding onto something of yours since that day, now where is it?
...
Tsugumi: Something of mine? (From that time?)
Claude: ... oh here it is, you were holding onto this and wouldn't let go 'until you collapsed from hunger', so I decided to keep it safe for you until you got better 'I seem to have forgotten about it, until just now', here you go.
Tsugumi: ... ah, wh, wh , WHAAAAAAAAAAAAT!? Is that, it can't be... right?
Claude: why so surprised? Did you remember something?
Tsugimi: ...uh, n, no, no it's just that, um it surprised me that there was something like this, yeah that all.
Claude: ok, keep up your good work of looking after the ojou, it won't be long now before we find something out about that Shuuheigumi brat 'hw pisses me off so much that even see him in my dreams, GIVE ME BACK MY OJOU YOU STUPID BRAAAAT!'
Tsugumi: (closing the door behind while claude cries) aaaahhh, no way this, somethimg like this how can it be? How will I explain to ojou? and how am I going to face him from now on.
(Starts fantasizing and proceeds to start blushing)
Tsugumi: AAAHHHH, no, no, no what am I fantasizing about? how could I? 'although...' no, this isn't the time, I need to figure out how to tell them.
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This is taking into account thay she finds out about the keys and the promise beforehand.
