I am not sure what exactly your question is about, but I still hope one of the following will answer it:
When someone adds your AragornisAwesome character as a friend, this will be the only character of yours he can see coming on/off line. You can play on your LegolasisLegendary alt safely, no one will know. Unless they add this alt as a friend as well, but they will never know if you never tell.
When you mark your character as "anonymous" people will not see when you are coming online or in which area you are at the moment. Though they will still see the time when you last logged off on that character.
Ishtarien - Captain 100
Ishtarel - HNT 100 / Ishtaridas - LRM 100 / Cabernetta - GRD 100 / Ishtari - RNK 100 / Ishti - BRG 100 / Lunasa - MNS 100 / Medovinus - BRN 100
The Mellowship - [EN] Eldar -> [EN] Evernight