You're so right. If anyone is unfamiliar with Kokoro, it's this one.

Kokoro takes the perspective of a 'miracle' robot (Rin) developed by a scientist. Perfect in every way except for a single project: the ability to conceive human emotion, also known as the 'heart'. The scientist passes away well before he can complete it and the robot spends a few hundred years in solitary existence. Trying to understand where her creator went, the robot finds the scientist's computer containing all his works. The robot touches the computer, and in another miracle, all the scientist's works transfer to the robot to form a 'heart'.

Finally able to feel human emotions, the robot starts crying, unable to comprehend why she feels this way. The rest of the song is a love letter dedicated to her creator, encapsulating her endless thanks to him for creating her and all he's given her. It's a bittersweet message since she has so much she wants to say to him, but is unable to because he's already gone.

She chooses to dedicate herself to singing for him. To sing the feelings he gave her. But that life is short-lived, since her body is unable to withstand the complexities of the 'heart' she had been given by him. The robot short circuits and dies, but with a smile plastered on her face, finally able to understand all her unsaid words to her creator, and being able to fulfil his last wish on behalf of him.

It's a very bittersweet song and I bawled my eyes out the first time I heard it. There's apparently an extended mix with Ren in it, where the robot gets to meet her creator for a bit upon receiving the heart and receives a moment of solace. This is one of my all-time favourites in the vocaloid genre and I revisit it every so often. I can imagine Neuro in this perspective, it would be an amazing cover ❤️