Harry: Harold, he never goes by his real name, but you love it. It just does not suit him well so everytime you call or text him you call him Harold Styles.
Louis: Tommo, from the megamind where he introduced himself as Louis the Tomo Tomlinson and it stuck.
Niall: Lepurchan, because he's kinda small like a lepurchan and he's irish. So it makes sense.
Zayn: AmaZayn, you just thought it was super funny so it stuck.
Liam: Cheesy Head! Because when he was a kid that was his nickname, Liam has never liked it but you use it anyway.