I wanted to do this in another DC forum I joined, but it's of a lower priority there... so I'm gonna do it here 3nodding

Here is a list of all the characters appearing in the keyhole on the back cover of each volume of the manga:

1 Kudo Shinichi
2 Mouri Ran
3 Mouri Kogoro
4 Megure Juzo
5 Agasa Hiroshi
6 Yoshida Ayumi
7 Yokomizo Sango
8 Suzuki Sonoko
9 Kojima Genta
10 Hattori Heiji
11 Kisaki Eri
12 Tsuburaya Mitsuhiko
13 Suzuki Ayako
14 Kudo Yukiko
15 Hattori Heizo
16 Kaitou KID
17 Okino Yoko
18 Haibara Ai
19 Toyama Kazuha
20 Takagi Wataru
21 Sato Miwako
22 Kudo Yuusaku
23 Shiratori Ninzaburo
24 Araide Tomaki
25 Kyougoku Makoto
26 Vermouth
27 Yamamura Misao
28 Hattori Shizuka
29 Megure Midori
30 Hakuba Saguru


That's as many volumes as have been released in English so far... I'll continue to update as I get more of them ^_^