Right after Kallen has liberated Lelouch from Pendragon, as she's flying back to the Ikaruga on the Gawain. She notices how peaceful he looks and just how much stress he has to carry as Zero. In a weird way it gave this troper a strong WAFF.
Almost all of Kallen's interactions with Lelouch in general. The strength of their relationship, even eight years after their separation, only seems to have grown instead of deteriorating. To the point that Lelouch outright admits to her that other than C.C., she is the only person in the Black Knights he completely trusts. Enough to trust her with his back, both to protect him and to take him down should he ever fall from the right path.