Just a quick note regarding Rosalina's recovery, since I feel edge guarding is where DK may shine in this matchup.
Be aware that she has three methods of recovering to the edge with her up-b. The standard diagonal arc, the reverse arc by hugging the stage and sliding up the slant, and the b reverse up-b-- which sends her directly upwards. Scout which one of these she will be using based on her positioning, and react accordingly. B-air stage spike may work wonders in the scenario where she opts to recovery low. This is due to the fact that her standard up-b has no hitbox! So challenge it!!
If she drops extremely low, you may be able to judge the usage of the b reverse vertical recovery. In this case, a well timed double jump rising d-air will solve matters.
Many Rosalina's will opt to mix in recovering high given they realize the dangers of committing low as well. So be sure to hold your ground until they commit first. If they do go high, save your double jump and punish with up air.