Katie said: 'All I saw was a handsome guy. We're perfect for each other because we both had the same troubles growing up.
'We're both size five, so we even swap our old clothes our mum's bought us but we hated.
'We look so convincing as a boy and a girl, nobody even notices now. We secretly feel so good about it because it's the way we've always wanted to be seen.'
High-school student Arin undergoes testosterone shots to give him the more masculine shape he's craved his whole life.
Meanwhile high-school graduate Katie, 18, has finished her female hormone course - oestrogen shots - which helped her to develop natural breasts with no silicone implants.
Katie, who has legally been a female since she was 15, believes she was born with naturally high levels of oestrogen and was teased as a boy throughout school for having small AA-cup breasts.
An anonymous donor was so touched by her story they paid for her £25,000 ($40,000) gender reassignment surgery just four days after her 18th birthday on May 16, 2012.
Katie added: 'Even from age three, I knew deep down I wanted to be a girl. All I wanted was to play with dolls. I hated my boy body and never felt right in it.
Boy meets girl, girl meets boy: Arin Andrews, 16, left, was once a girl called Emerald while his girlfriend Katie Hill, 18, right, was born a boy called Luke
Before the transgender treatment: Emerald Andrews, pictured left at the age of four at dance recital in Tulsa, Oklahoma, before she became Arin, and Luke Hill pictured right at the age of five before he became Katie