Nina. I like how short and snappy it is. It's soft but not to the point of being mushy. To me it fits someone a bit eccentric.
Helena by comparison is boring and at the same time over-complicated (I'd still prefer
Nina over
Helen but not by as much).
That said I think
Helena and
Nina would make a great sib-set! I love them together in that context.
https://www.behindthename.com/pnl/109883