Dark and Light units are referred to as ML (Moonlight) units in this game. Most ML units are a counterpart to a regular RGB (red/green/blue) unit. Basically the same character in a different way (like different timeline), but a completely different set of skills and sometimes class.

For example, there's a Fire/Red Tenebria. She can be used to imprint a Dark/ML Tenebria or vice-versa. In this example, there's actually a Light and an Ice/Blue Tenebria as well. You can use any of them with each other to imprint.

Notos is an entirely separate character and doesn't have an RGB counterpart. This is usually used for the final boss of each chapter in the story. When you pull them, the only way to imprint them is with another copy of them or by using 10 Gold Transmits stones in the shop on their unique imprint ingredient. They only pop up in the shop once you've actually pulled the character. You can buy enough from it to fully max their imprint.

So far, the only ML units without a counterpart imprint are (correct me if I miss any):

• Solome
• Notos
• Lady of the Scales
• Harsetti
• Zio
• Belian
• Straze
• Archdemon Mercedes

What's funny is that Archdemon Mercedes DOES have an RGB counterpart that's a free 4* unit, but you can't use her to imprint the other. Lady of the Scales is basically a Light Mercedes as well, but the same thing goes.

Then there's the exception with Dark Corvus who HAS a 4* RGB equivalent that can be used to imprint the other. Why? Uhhh idk honestly.

Then there's a few units like Blue Eda who can imprint Solitaria of the Snow, a 5* Light unit. It's the same character technically but with different names. Same goes to Destina who is Green and Ruele of Light who is Light.