Cannot Spit It Out: He cannot tell Riku that he is Dark in fear of getting rejected because in order for him to stop transforming into Dark, Riku needs to love all of Daisuke, which means both Daisuke and Dark — who Riku absolutely... hates.
Identical Grandson: Daisuke looks a lot like his grandfather did in his youth, enough to cause at least one person (the spirit of a girl Daiki befriended when he was Daisuke's age) to mistake him for the former.
Secret Identity Identity: Considering Dark's always telling him "I am you," it isn't too much of a surprise that in the ending chapters of Stage 4 during the dance, Daisuke's and Dark's emotions seem to be getting melded together, and Daisuke is having trouble figuring out just who is dancing with Risa and just who is feeling these things for her. This is an interesting case, considering just how independent from each other the two are generally portrayed.
Sexier Alter Ego - In the eyes of most of the girls in the strip, Daisuke is cute but Dark is superstar level hot. Riku is the notable exception who prefers Daisuke.
Oh so much. Dark's love interest (in the manga) Riku totally hates Dark's guts, and Dark lets down his second love interest Risa because he knows he can't be with her. Because Dark disappears whenever his Niwa host falls in love and their love interest reciprocates, Dark suffers whenever he falls in love because he can never stay.
Daisuke's grandfather Daiki reveals to Kousuke that while he was Daisuke's age, Daiki and Dark were both in love the same girl (who eventually becomes Daisuke's grandmother), but she chose Daiki over him, causing Dark to disappear.
Ambiguously Bi: It's hinted that he may have feelings for Risa. In the anime, it is also hinted that Satoshi may have romantic feelings for Daisuke. When this is brought up, he doesn't openly confess to anything, though he never denies the accusations either. If anything, he encourages the idea, which has led to the source of much confusion regarding his motives. It has even been stated outright by Dark that Daisuke is the one who causes Satoshi to turn into Krad. Krad himself has also hinted at Satoshi possibly wanting more than a friendship from Daisuke at least once in the anime, and more than twice in the manga. In any case, he at least wants to be close to Daisuke despite the danger of Krad to both of them.
Rascally Rabbit: He acts as a familiar to a Phantom Thief and his shapeshifting powers are used to trick people. In the anime, he also sometimes engages in minor acts of michief such as tagging along with Daisuke at school by hiding in Daisuke's school bag or eating all the strawberries.