In Jain Dharma, a Siddha soul is separated from all other souls by this key fact:

It has completely destroyed all karmas and become fully liberated.

A Siddha is a soul that has:

• No karma left
• No body
• No rebirth
• No suffering
• No attachment or aversion
• Reached the top of the universe, called Siddha-śilā

What makes a Siddha different?

Compared to ordinary souls, a Siddha has completely achieved:

• Keval jñāna — perfect, infinite knowledge
• Keval darśana — perfect perception
• Ananta sukha — infinite bliss
• Ananta vīrya — infinite energy

Important point

A Siddha is still a pure soul, but unlike other souls, it is:

• free from matter,
• free from karmic bondage,
• and permanently established in its true nature.

So, what separates a Siddha soul is complete liberation from karma and embodiment. It is the soul in its absolutely pure, perfected state.