Is it rather a feeling than an emotion?
"the feeling or state of being unable to do anything to help yourself or anyone else"
"Inability to defend oneself or to act effectively."
The same source also provides example sentences, the first of which is ‘worst of all is the feeling of helplessness’ - exactly.
The definition uses "defend", which implies that there is something to defend against - that which inflicts, in most cases, pain or other significantly undesirable outcomes.
I would suggest to extend the definition from "oneself" to "oneself or others".
Witnessing Pain
Indirect Traumatization
missing in many systems to categorize emotions
Helplessness is inherently unconstructive.
Constructiveness
The repetition
The expression of this emotion is unconstructive.
... both as feeling during an experience as well as when re-experienced during an emotional narrative.
Emotional Narrative
(Overwhelming) Helplessness is a core feature of / inherent in any traumatic experience.
At the heart of all traumatic experiences lies the fact that one experienced an emotionally painful experience (which can be physically painful at the same time) with the inability to end, limit or meaningfully influence the situation, i.e. helplessness.
Trauma
Pain