Daisuke and Ken have been pining after each other for many years. They finally decided to take the first step not long ago and are still exploring their new relationship. Ken wants Daisuke to suck at the skin of the back of his neck where the Dark Seed had entered all that time ago, as if sucking out poison, because he's curious to know how that would feel. Unbeknownst to the both of them, this causes Daisuke to become infected with the Dark Seed and after a while he becomes a new Digimon Kaiser. 

Kaiser!Daisuke needs to have a reason to keep Ken around, maybe something like how Ken is his favourite distraction. Ken stays by Daisuke's side because he's trying to get him back to his old self (and distracting the new Kaiser would be a good thing).

Bonus: Make Kaiser!Daisuke wear hideous platform shoes (which may or may not have flames on them) so he'll be taller. I'd like Daisuke's Kaiser costume to be mirroring the style of one of the Chosen guys that Daisuke is a bit jealous of. For example, Yamato because he had assumed for a long time that Ken found him attractive.