Bhagavad Gita

ನತ್ವೇವಾಹಂ ಜಾತು ನಾSಸಂ ನ ತ್ವಂ ನೇಮೇ ಜನಾಧಿಪಾಃ ।
ನಚೈವ ನ ಭವಿಷ್ಯಾಮಃ ಸರ್ವೇ ವಯಮತಃ ಪರಮ್ ॥೧೨॥
natvevāhaṃ jātu nāSsaṃ na tvaṃ neme janādhipāḥ |
nacaiva na bhaviṣyāmaḥ sarve vayamataḥ param ||12||

Gist of the sloka:
I was not in a state of non-existence. So is yourself and these kings. Even in future we would never cease to exist.
Lord Krishna is very clearly indicating to Arjuna that all [souls] existed before and will continue to do so ever after. They also continue to be present now. All souls are immortal.
Soul is not destructible and whereas body is meant to be destroyed.
Each soul has its own ‘swabhava’ which is attached to itself. It cannot be changed or modified. Lord rather seeds the universe with these souls and allows them to grow/evolve according to their basic nature/swabhava.
He is like a farmer who plants seeds into the land. Farmer allows each seed to grow according to its nature. He only nurtures them, while not showing any partiality. While each soul has its individualistic character, each is different from each other. This is the reason there is so much variety in the world. It is true to the minutest level wherein each leaf in the same tree is different from each.