Ok, i havent used Rippers yet, but I just had a quick look at the Rule book.
Page 99: combined units section: Characters and the unit he joined (including any other characters in the unit) are treated as a single combined unit for ALL rule purpose, save for the exceptions listed (and these do not include bloth toads).
SO!!! this is my interpretation. If you put it on a unit with a character in it (which is considered a single unit) you actually automatically put it on both the unit and character. Therefore, Should the character leave the unit it actually should take the toad with him (while it also remains on the unit), so you should get a second toad marker. If he then joins another unit the marker should effect that entire unit as well as it is considered a single unit with the character.
Now, this will probably get you into an argument with your opponent, but this what the rulez say.
Anyway!
-You dont have to specify, in case of combined units, where you put it cause it is considere a single units (just like spell effects affect the whole unit, including characters)
-It definately should work on both the unit and the character that are in the unit when the marker is placed.
-It should also work on a new character joining the unit during the game
-it should also work on the original character in the unit when he leaves. (Spell effects also dont go away when a character leaves).
-it should also work on a new unit that the toaded character joins. (although i would not make a big argument over it if my opponent objects), but definately still on the character.