WARNING: The following contains spoilers for Episode 12 of Girlfriend, Girlfriend, "Girlfriend, Girlfriend," now streaming on Crunchyroll.
The season finale of Girlfriend, Girlfriend comes about with a big reveal from Nagisa, providing viewers with her full backstory and additional insight into her questionable actions in the previous few episodes. Nagisa is the newcomer to Naoya and Saki's relationship and seems to sacrifice a lot to make Saki happy in her position as his first girlfriend. The season finale finally has Nagisa confide in Saki, and what she shares is very illuminating.
Still sequestered at the hot springs with Naoya, Rika and Shino, Nagisa and Saki get a moment alone together amidst the hustle and bustle. Nagisa takes advantage of the brief moment of quiet and starts to tell Saki how she first met Naoya, and how this made such an impact on her life.
Nagisa tells Saki about seeing them before their entrance exams for high school -- Nagisa even witnesses the moment when Naoya makes the deal with Saki to become her boyfriend if they get into high school together. The way Naoya was completely devoted to Saki inspired Nagisa and she immediately fell in love. From that moment on, she dedicated herself to becoming a girl worthy of Naoya's love. She gets a part-time job, learns to cook food that he would like, works out excessively and changes her appearance -- all for Naoya.
She then explains why she is willing to give Saki her full support in their relationship. Saki needs to remain happy in their weird situation -- otherwise, Nagisa will get the boot and won't be able to be with Naoya anymore. This is why she puts Saki's needs for Naoya's attention ahead of her own. It's out of her own self-protection; she really is willing to do anything for Naoya's love.
Unfortunately for Nagisa, Naoya overhears this entire conversation. He quickly tells her to stop holding back because her happiness is at stake as well. What follows can only be described as a long chase sequence, with Naoya and Nagisa somehow able to sustain a full conversation as Naoya tries to catch up to her. At first, Nagisa is reluctant to admit that her needs are important as well, and it quickly becomes apparent that Nagisa feels insecure in their relationship, which is why she is protecting herself.
As soon as Naoya realizes this, as he conveniently catches up to her, he starts to become aware of the full burden of his three-way relationship. The precarious balance that he so desperately asked for is more at stake than he realized, and he immediately makes it known to Nagisa that she is equally important to him as Saki. It's not a competition to him -- he just wants to fulfill his promise from Episode 1, when he vowed to make them not regret their decision to date him. He just wants to make them all happy equally.
While Nagisa's backstory is a huge reveal, the season finale ultimately ends on a happy note, with Naoya realizing just how lucky he really is. It hints at future trouble with Shino, who seems possibly taken with Naoya herself. Rika, of course, is still making her moves on Naoya, but his focus has shifted. He now sees Nagisa and Saki as his two beloved girlfriends, and he seems ready to do anything to keep it that way.