Now I'm really curious to know about Isanari and Marou.
As always with Negi, date is .... Seriously , what I hoped for, I wonder.
Itsuki "big trouble" burned in one page, that was even fastest than Negi's standards!
The art on the other side was very beautiful!
Isanari and Maruo were once classmates. That still doesn't justify Isanari making it into Maruo's office. The security letting him in (assuming) suggests that Isanari used to work for/beside Maruo in the hospital
You intrigued me, so I read all the versions currently on Mangadex. It seems that Isanari does not know how the office is made, moreover, in 3 translations out of 4 security takes him away. But I agree with you that it is strange how he got to Maruo's office.
My guess is that perhaps we could see the real father of the quintuplets in action: chapter 97 "there will be a famous speacker/teacher who will hold a seminar here", chapter 98 that strange question from Isanari to Itsuki "did something happen?"
It would be interesting to see which strange relationship links Isanari and Maruo and even more interesting to see the actual father of the girls!