The first OC up is from DGPrimal and that is....
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The Dark Chronicler Tristan

For Tristan's partner, I wanted to lean into his abilities such as his Mimic ability and also with his relation to the Void and darkness.

Alright, now going onto his Rookie. I have chosen it to be.

Copimon-

This is mostly in reference to Tristan's Mimic ability. Due to the fact that Copimon being compressed data that can transform into really any digimon. So I feel like it's the best pick for Tristan's rookie.

Moving onto his champion I decided to give him.

Devidramon-

This is mostly tying into again the Void and darkness that Tristan is so tied and related with. Devimon can also work here but I decided to use Devidramon to also bring in the fact that he has the boost gear and with that dragons.

Onto his ultimate I decided to give him.

Mephistomon-

Again this is mostly tying into the Void and darkness that he is related with. Nothing much more to it

Now for his Mega, I feel Tristan would have two separate Megas. Meaning his Copimon can became one of the two at any time. And such for his first Mega I gave him.

Gulfmon-

I gave him this again because darkness and due to the fact that of it is capable of destroying souls. Kind of like a Void

And for his other Mega I gave him.

Parallelmon-

Simply due to the fact of Tristan being someone who travels to other worlds. And Parallelmon is also capable of traveling to other worlds and drag others from other worlds to the world that it's currently in.
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So yeah that's what I feel Tristan's partner would be along with their Evolutions. Tell me what you think and tell me if you have anything that you disagree with. Ta ta~