It's really annoying, that one has to add every friend to each created character. That's why I've created the global friendlist addon.
With this global friendlist you have to add a friend only once, and it will add this friend to all your characters, with which you log into the game.
This works for already added friends and for all new friends you ever add.
If you log in, the addon compares the server friendlist with all the collected friends and adds automatically missing friends from your global friendlist. You'll get the standard chat information for the added friends.
If you added a new friend, you don't have to do anything more, because the global friendlist does already know this new friend. Next time you log into the game with an other character, the addon will add this friend.
You can also delete friends, the global friendlist knows this also. BUT to avoid killing the global fiendlist, you have to delete this friend in every character yourself. Because I think it should be seldom to delete a friend, I won't spend time on how to automatically delete friends from all characters. If you don't delete the friend on every character, the global friendlist will get him as new friend again and adds him automatically to your friendlist. ;)