pimpernel (n.) c.1400, from Old French pimprenelle, earlier piprenelle (12c.) and directly from Medieval Latin pipinella name of a medicinal plant. This is perhaps from *piperinus “pepper-like” (so called because its fruits resemble peppercorns), a derivative of Latin piper “pepper” (see pepper (n.)); or else it is a corruption of bipinnella, from bipennis “two-winged.” The Scarlet Pimpernel was the code name of the hero in an adventure novel of that name published 1905.
The OED seems rather unsure, “or else, otherwise, perhaps…. ” – Actually, the scarlet pimpernel is a variety of Anagallis arvensis, which are rather prosaic in form but rather remarkable in that they only open when the sun shines. Thus, the pimpernel hides in darkness, and one assumes this to be precisely the reason the Baroness Orczy chose this name for her fictional hero, Sir Percy Blakeney, aka the Scarlet Pimpernel. The hidden identity trope was later elaborated in the characters of Zorro and Batman – ineffectual and foppish aristocrats on the surface, hot cauldrons of seething swashbuckling heroism underneath. The Scarlet Pimpernel rescued French aristocrats condemned to the guillotine – revealing the Baroness’s own partiality for the aristocracy. Although she was no slouch in her own right, using her pen (rather than the sword) to transform herself from relatively penniless Hungarian aristocrat to the owner of an estate in Monte Carlo.
My favourite incarnation of the Pimpernel is Leslie Howard (“Kiss me one Ashley, and I’ll live on it the rest of my life”) in the 1934 film:
“They seek him here/they seek him there/those Frenchies seek him everywhere/ Is he in heaven?/is he in hell?/that damned, elusive Pimpernel.”