I like Agatha. I like Aglaia (Agh-LIE-ah) even more www.theoi.com/Ouranios/KharisAglaia.html
Why are people on Mumsnet so hung up on nicknames? I've never, ever been called by anything other than my full name, even by my DH and my dearest friends, except by my sister, and that's a private pet name used only by both of us. It's really not compulsory to have a nickname...
Tig/Tiggy as a nickname just reminds me of royal nanny Alexandra Legge-Bourke - but - ye gods! - there have been so many vicious and nasty things said about her. I have absolutely no idea whether any of them is true, but the whole atmosphere of royal rumour-mongering, for and against, dating from that era is deeply, deeply unsavoury. And probably very much exaggrated to please the press. Ugh. But I still would not use 'Tig' as any kind of name.
Continuing the royal theme, was 'The Tig' not used by Megahan Markle, before she met Prince Harry, as the title of her lifestyle blog? Named after a wine she liked, allegedly. Am not being judgemental - there's nothing wrong with writing a blog. thetigarchives.tumblr.com/
But if you are wanting to use a nickname, perhaps best to research earlier uses???