Enfys Nest is the masked leader of the Cloud-Riders swoop gang, played by Erin Kellyman in Solo: A Star Wars Story (2018). StarWars.com introduces her the way the film does — as a threat whose "mask and armor hide all clues about the Cloud-Rider chief's identity and background", with motives that "are also a riddle".
The reveal that reframes the film
The riddle resolves about two-thirds of the way through: the marauders raiding Beckett's coaxium jobs are not pirates but a resistance, robbing shipments that would otherwise fund Crimson Dawn and the Empire, and their fearsome leader is a teenager. Everything the audience has been encouraged to assume about who the villains are gets rearranged in one scene.
Proto-Rebellion
Her function in the canon is to sit at the point where crime and insurgency are the same activity. The coaxium Han hands her at the end of the film is explicitly seed capital for a rebellion that does not exist yet — one of the clearest statements the saga has made that the Alliance was assembled out of smugglers, thieves and people with nothing left to lose.
Design
The helmet and the electroripper staff were built to read as menacing from a distance and improvised up close — salvage, not manufacture. It is a rare Star Wars costume that tells you the wearer's economics before you know anything else about them.