Ok I don't know if Niall is actually dating Barbra but if she is this is what I would say to her:
Dear, Barbra
You have gotten the man most of us would dream of having. which means you are the most lucky person in the world!
I would never have thought I would be happy to see Niall taken, but with you I am! I'm glad that he is happy. that is what most of us want. others may give you threats or call you names. just disregard that, your beautiful.
I have been thinking of crying for the past few days since you where with Niall on New Year's Eve. I never cried, for Niall got to kiss someone he loved dearly. it may have not been me or other girls but at least he is happy with you.
This letter may be running a bit long so I will get to the point. Barbra you have been chosen to be Niall's. which means you are the most lucky girl on earth! Hold him tight, don't let him just slip through your fingers. and when you do hold him tight be carful for you are holding my and a lot of other people's world. don't let him go. do whatever you can to keep him.
When you kiss him, kiss him softly
When you hug him, hug him tight
Please don't hurt him. Niall James Horan from Mullingar,Ireland is my entire life! and trust me, a lot of girls want to be where you are right now, in Niall's arms.
