Demetrius is comforted by Raidyn's quiet nature. He's not energetic, not rambunctious, and certainly not always trying to ruin Demetrius' day. Because of this, they get along quite well, and they enjoy each other's company in silence. Demetrius appreciates his adopted brother's fears, and is always there to help him through them, as well.
Aside from Riggs, Raidyn seems to get along with Demetrius the best. Maybe it's because they're both overall grumpy and somewhat sad mothkittens, Raidyn's not sure. But he can always relate to Demetrius in some way or another, especially when it comes to Lysander's rambunctious behavior. Often Raidyn can be found talking small talk with Demetrius, and being comforted by his adopted brother's silence.