When I went through the manga it seemed to me that:


Who knows about Ryouga:

1. Ranma
2. Shampoo
3. Cologne
4. Genma
5. Chinese Guide
6. Plum
7. Herb, Lime, Mint
8. Pantyhose
9. Soun (possibly)
10. Mousse
11. Akari
12. Happosai (possibly)
13. Anyone I left out?
14. All of the Phoenix people I think...
15. Azusa

Who doesn't know:

1. Akane (duh)
2. Kasumi
3. Nabiki (she woulda been blackmailing him for ages)
4. Nodoka
5. Ukyou (see the Cave of Lost love manga, I don't think she 
realizes)
6. Tsubasa
7. Konatsu
8. Soun (possibly) (he's too stupid ^_^)
9. Happosai (possibly)
10. Kuno
11. Mikado
12. Just about everyone else!

Hope this helps (and is right!)

misa