Registered via internet if this helps.
This is a glitch! Look in the passerby and I am 100% sure that you will see your friend, it has happen to me many times that when I add someone they appear as passbyer.
Hope I helped :D
I had this problem with my DS earlier this year, the method I used was getting that person to delete my Friend Code and add them again and you should be good to go.
TL:DR: Delete the person and add them back.
Hope I helped!