A.N. So I am now the owner of DVD's of the anime and managed to get the manga from the library, as I was reading it I noticed that after Dark stole the fallen angels love potion and kissed Riku to make her be quiet the potion spilled on them while he was kissing her. the next day Daisuke starts to notice Riku, coincidence? I doubt it.
Fade
He could tell that the love his partner felt for Riku was fading now without her constant presence to reinforce it; more often the mental images that slipped across to his mind were only nostalgia and friendship, even the flaws in her character were less glossed over. He had worried that her stubborn refusal to accept that Dark had been part of Daisuke and losing him had hurt, would hurt Daisuke irreparably. Fortunately she had just avoided that topic of conversation. One day he'd asked Daisuke when Riku had become his trigger instead of her sister, he knew it had been Risa once after all. The answer had made him wonder if there had ever been proper love there at all: certainly the fact that Daisuke had abruptly started noticing her after Dark had stolen a love potion and lost it on her balcony, and she having glimpsed Daisuke for a moment had tried to return the empty vial the next morning was indicative. Especially given that he knew for a fact that at the time Dark had stolen the vial it had been full.
Loneliness
I had never thought Daisuke would understand the loneliness I feel when watching normal people chat freely without a care, and I know I could never be included in that kind of life. But he understood best, having grown to feel the same due to the veil of secrets that hide the knowledge of such things as: what Dark is and why he stole, or why the two of us have such uncanny knowledge of every museum, and are always alert when visiting one. He understands more and more with every time we meld and pull away again, for every feeling is shared then, as is every memory, and we come apart with knowledge of each other that is deeper than blood and stronger than steel. It took me a while to realize it but now even when I am by myself I'm never alone, not with the warmth of Daisuke's mind always at the back of my own.