Nell is an shortened version of Ellen, Ellin, Eleanor, Ellenor, Elinor or Helen and I have seen it registered (baptism/birth) as a name in its own right. Sometimes it is affectionally lengthened again to Nelly/Nellie.

If you are looking for the most likely original christian name of someone later called Nell, Nell is most commonly the shortened version of Helen as Ellen/Ellin tends to be the normal shortened version of Eleanor etc with Ellie as the affectionate version. There again, sometimes people were called by different names later in life to prevent confusion if there was already have been someone of their name in the community, or she reminded friends/relatives/employers of another Nell.