When you add someone as a friend, add a little option that makes it so you can add a note for them and when you hover over them it shows up. Just something to keep track of the many friends you can make on Osu!
From what I understand of OP's post, this is not what he requested. He wants the ability to attach a little memo for each of his friends, only viewable by him. To remember when he added them, or how he met them, etc.
Personally I don't see much interest in it. You can always create a local Word or Excel document by yourself to keep track of your friends if you really need it
Did you read my post there properly? It is to support a short note of up to 150 characters and I even photo shopped how the "little note" behaves when the user hover the mouse over.
Anyway it's up to mod whether this is a duplicate or not. The idea described in the OP has already been discussed in the quoted thread.