Black Zetsu shouldn't have emotions. It was the will born from Kaguya Otsutsuki to revive herself. Its birth was for one purpose only.
However, at this very moment, its mood was exceptionally complex. It was sad and happy, a great sorrow and a great joy that was almost too understated. Though it had always been deceiving Emiya Shirou, it was still touched by him. He was apologizing to it, his companion, for his failure. Perhaps... this is the feeling of being seen as a companion?
It knew Shirou sincerely considered it his companion. The path to reviving Kaguya was too lonely, and its deception had only brought it the sincere Emiya Shirou. This ancient person who believed in the God Tree and the Rabbit Goddess Kaguya seemed so precious in this era that had forgotten them. So precious, like a spark of fire in a millennium-long cold, dark night. The only spark that had ever made it feel warmth.
What a great genius Emiya Shirou was! Yet at this moment, he had become so dejected because he had lost hope of reviving Kaguya. This dejection shouldn't appear on him. The Shirou of the ancient era was omnipotently powerful, capable of standing shoulder to shoulder with gods. This genius had once nearly changed the world. But now, he was acting so dejected, and even while dejected, he still had to apologize to his companion.
If its mother Kaguya knew of his experience, she might also shed tears for this naive and pure person.
Of course, Black Zetsu's heart was also very joyful. Because the thing that made Shirou most desperate was, to it, a trivial matter.
"No need to despair over these small problems," an irrepressible, strange smile appeared on its face. It began to comfort Shirou in a hoarse voice. "Actually, the answer you're looking for... has been by your side all along."
As it finished saying this, its heart suddenly sank. Wait... what nonsense am I talking about? What am I doing here? I should have come to make him keep secrets, how did I end up leaking more secrets to him!
Although Shirou was trustworthy, its identity was the most important secret. How did it say it in a moment of emotional excitement?
Shirou gave it no chance to react. Upon hearing the words, he had already suddenly raised his head, looking at Black Zetsu with a face full of shock. "Black Zetsu, could it be you..."
Black Zetsu didn't have time to think. It couldn't even think of who to pretend to be. In its heart, it cursed itself for leaking the secret, while also comforting itself that Shirou could be trusted. In this conflict, it revealed its true identity. "That's right. I am the will of the Rabbit Goddess Kaguya."
After revealing its identity, its mood also became completely dead-hearted, just like Shirou's had been a moment ago. It could only choose to trust him. Just as Shirou, in his despair, could only trust Asura. Even if it regretted it, it was too late. It had already leaked too many secrets. Its true identity... maybe Shirou could have guessed it one day. Having exposed its desire to revive Kaguya was not much different from exposing its true identity.
It doesn't matter if I said it... Emiya Shirou is definitely trustworthy!
"Wait, this..." Shirou looked in disbelief at Black Zetsu, who had just directly revealed its identity, his face as if he had just seen something impossible.
Wait... what are you doing! I just wanted to casually trick you into trusting me... how did you reveal your entire background! Has this fellow learned nothing in a thousand years? Isn't this too easy to deceive?
He remembered back in the ancient era, he had shouted about wanting to destroy Ninshu, and this fellow had later also clamored that Ninshu must be destroyed, leading to Ninshu actually being destroyed by him once. He had just wanted to use Black Zetsu's desire for revenge to gain its trust, but it was really led astray. He lied about believing in the God Tree and used Kaguya to lure it, and it revealed its idea of wanting to revive her. And now, he made up a few heartfelt words, and it revealed its true identity again.
It's been over a thousand years! How is this fellow still so easy to trick? Shirou silently criticized Black Zetsu while also wondering if he had tricked too many secrets out of it back then. It was too sincere; he himself felt a bit embarrassed. He didn't even dare to imagine what Black Zetsu would be like in the future when it learned his true face. It would probably not be in a very good mood.
Now, Black Zetsu was still proactively apologizing. "I'm so sorry. All this time, I have hidden my true identity."
Yes, it still had to explain clearly. It even needed to make up a story, one favorable to Kaguya, because Shirou was a completely kind and good person.
"No, wait," Shirou felt a bit unable to bear the apology. He was worried that accepting it might lead to some overly strange changes in Black Zetsu's mentality in the future.
However, he realized another problem. His past self, who had always been sincere, would surely not be able to immediately forgive the Black Zetsu who had hidden its identity from him, even if its identity was the will of the Rabbit Goddess he believed in. Directly forgiving it would seem suspicious. He should be confused and struggle for a while. At least put on an act of being unable to bear the shock, and wait for it to try hard to ask for his forgiveness. Only then would it trust him more. People believe what they have worked hard to obtain.
Communication between people is really difficult.
Sometimes it's not easy to handle interpersonal relationships well.
"Wait a moment," Shirou waved his hand weakly, as if his mind had received a huge shock. A wounded dejection appeared on his face, as if he couldn't accept that his former companion had deceived him. "I'm a bit... surprised. In any case... let me think alone for a while."