Okay, I'm trying to decide between either Dirty Water / Magazine or Mercury / Cleaning Cloth.
My reasoning is as follows.
1. Our first clue, and arguably our best one, is the method of murder (Poisoning). Right away we have eliminated Mia and Yugi as potential suspects, and tightened our list to Remiel (Radiation, Powder drug), Madame Professor (Poisonous Gas, Red Wine), Rapt (Venomous Snake), Christa (Alcohol), Mintprincess (Chemicals, Mercury), and Phoenix (Dirty Water). Like I said, if it were me, I would have said Illness/Disease for Dirty Water (everybody knows you need to boil it!), but perhaps Ryven wasn't thinking that way.
2. Our last clue, Duration of Crime -- for Days, limits that down even further. From that list we can eliminate Poisonous Gas, Red Wine, Venomous Snake, and Alcohol, as all of those would take much quicker than a few days to kill somebody. Radiation could take conceivably longer, so we'll eliminate that as well. That leaves us with Remiel (Powder Drug), Mintprincess (Chemicals, Mercury), and Phoenix (Dirty Water). I know I'm not the spy, so that leaves us with just Mint and Phoenix.
3. Our second-best clue is the Bookstore. Ryven had about 20 locations to choose from, but he chose the Bookstore. Why? That suggests, to me, he was honing in on either the Dictionary, the Magazine, the Gift, the Confidential Letter, or possibly the Cleaning Cloth. If I had to pick a location for the Cleaning Cloth, I'm not entirely sure what location I would pick (a mansion, perhaps, or dry cleaning service), but perhaps the Bookstore would be in my top five.
4. Fishy could refer either to the Dirty Water, if we interpret it literally, or to Mercury, which, as Phoenix pointed out, is commonly associated with fish. I really don't feel that it applies to any other clue.
5. Perverted is a hard one to interpret, and suggests either the Wig (Remiel), Stockings (Christa), or Magazine (Phoenix), but again, if we make a leap of logic and associate Perverted = Dirty = Needs Cleaning, it might also suggest the Cleaning Cloth as well.
6. I'm really not sure what to make of Morning, other than most people do their cleaning in the morning. But it's also possible that it might refer to Dirty Water, as in when most people get up to use the bathroom?
7. The Nothing clue doesn't really help us eliminate anything, so I'm going to ignore it.
8. Love would seem to draw us away from either suspect, other than perhaps someone spends a lot of time cleaning something that they are proud of? Also, Ryven covered it up when I said I was leaning back toward Dirty Water / Magazine, so perhaps that's a good indication of what he meant.
All right, I'm going to go ahead and use my guess for Mintprincess with the Mercury and Cleaning Cloth.