Well, you usually translate helpless with one of two words:
If you mean "defenseless", you can use "indefenso" (or "indefensa" for words with feminine grammatical gender).
If you mean "powerless", you can use "impotente" (same for masculine or feminine words) but just take into account that "impotente" is used a lot more frequently in the Spanish world for referring to a type of male sexual dysfunction. There's nothing wrong with it, just be aware of the context it's being used.
Hope this helps!