This is Lloyd ZX, from the spinner set 9574. It seems that he's in demand, because within days of being available at shop.LEGO.com early in September, the set went onto back order until mid-October.
I managed to find one in my local Tesco, in a newly opened box of the new spinners.
Can the set be found on the shelves where you are, or is it disappearing fast?
I believe he's been hyped up in the cartoon series, which might explain his popularity, although as it's not aired here in the UK, I haven't seen it to be sure.
He's a nice figure. The green spikes are very cool, and uncommon. His spinner is attractively decorated and if you only buy one Ninjago spinner (perhaps before they are retired next year (maybe)), make it this one.
Of course, he's not the first green ninja: the other two in this photo are from 3346 from 2000, which was a hard-to-find set then and much in demand now. New examples will set you back $30/£20 each on BrickLink, while a MISB set will cost you around £80. Unbelievable for a set that cost just $5/£5 twelve years ago!
I suspect the rarity and coolness of them is a contributing factor to the high demand for Lloyd ZX.