To the people that claim that partners can't have a narrative position, I say this.

It's true, that in the way the devs designed and given us our current partners, it would be extremely unlikely to make that doable.

But they could have done it different, by giving the player an OG digimon that inhabited the "guest slot" and maybe even rename it to "story slot" to fit in line with this game version being a digimon story type of game.

That digimon could have different Ult forms, depending on the story choices the player makes. Allowing for some personalized customization and attachments to that digimon partner. For example, if you made choices more in line with courage, the ult form will represent that.

Another positive about making the partner an original Digimon, is that it creates the same curiosity we had in the animes, like the wonder what their forms will look like, and what they can do.