It's not really the Welsh form of Helen. The name 'Elen' is rare. It's rather old and originated in Wales from their word 'elen' (nymph). It's my name, so I did good research on it.
Both Elen and Elin are Welsh forms of Helen, although the latter is more common. The eldest daughter of Llywelyn Fawr (Prince Llywelyn the Great, 12th-13th centuries) had the endearing half-rhyming name Elen ferch Llywelyn.
Sorry, just to clarify, what I meant to say was that Elen and Elin are both *Welsh names*; quite possibly the derivation of Elen is from the word for a nymph, as moonlet says, and not from Helen -- I'm no etymological expert.
I really like this name, partly because it's my name. Also, I find that it's quite unique, as I've only ever met one other Elen. I've met plenty of Ellens. Also, I find that it can be quite annoying if you have English relatives or friends who call you Ellen. Plus, I have friends who can say Elen but they say Ellen.