when kid Rena said she'll be an example for the others sister... Itsuki is the last one to come to mind. I mean what did she do? took upon her self to act like the mother and try to take school seriously although failed hard at it enough that she needed a tutor.
on the other hand why did Itsuki appear the first time in front of Fuu-kun just to tell him "goodbye"? and now why does she have a 180 turn? because her sisters like him? don't make sense man, if she became jealous then why'd she even say goodbye in the first place? "No... i don't want anymore cake"... eats the entire cake.
The first appearance of adult Rena seems more in tune with Yotsuba as recently she said she'll support him going forward. You could also think of Yotsuba as acting as an example for her sisters as she's the most independent one and goes about helping most other people. But Negi built her up to be bad at lying and acting, then again she did have a moment where she said to Fuutarou "it's because i like you" and then was like "see, i can do it too". Did she mean lying, i mean she's a quintuplet so she should be able to do what the others do, on the other hand if she's as Negi build her, maybe she was able to say it because it was the truth. Honestly so far it looks like there isn't a person Yotsuba wouldn't like, she's really a people's person.
what are you doing Negi, what's you smoking, pass me some of that.