Hahaha...Negi really pulled a surprise mechanic on us.
But, if you really think about it, it was always Yotsuba who's been silently watching Fuutarou since their second meeting in high school. She's also the first one who welcomed him quite frankly when Futarou became the quint's tutor. You see, compare to the other sisters, Futarou didn't put so much effort to get close to Yotsuba. She even helped him to ask her sister to be tutored by Futarou.
Also, in festival arc-yotsuba's side, it is implied Futarou knew who is Rena. For him, Rena is his savior and first love. Because of her, he didn't feel his life to be empty anymore and didn't end up to be a delinquent.