The first question is how to find your victim. They can be located on fandoms, fan fiction sites, and Scarlet Pimpernel forums. The Scarlet Pimpernel is the key which has made their accounts so insecure. I did some research, and he appears to be an insane 18th century superhero. Don't question their sanity, just be grateful for the chance to steal their identity.
Then there is the simple matter of their password. The picture of ease, as it will either be something like pimpernel, scarlet, or I3Percy. If the password requires numbers or symbols (for higher security), it's pretty much guaranteed to be '92. Blakeney, Marguerite, Margot, Cravat, or hateChauvelin are also popular. If they intend to dupe you with a picture password, where you have to draw a design on a photograph to get in, the task is no harder. If the photo is a red, five-pointed flower, trace it. If it is a picture of a ridiculously dressed man, perhaps with the caption Sink Me! underneath, trace the cravat, (that's the necktie-thingie) and draw a heart around him. If it's a picture of a girl with an absurd amount of hair, also draw a heart around her and trace that star-shaped flower in her hair. Their pin is 1792. Do not doubt. They are really that stupid.
If that didn't work, chances are you've hit on one of the more creative ones. Try Dewhurst, Hastings, or Ffoulkes. Still no? Zooks, Zounds, SinkMe, M'Dear, or Odd'sFish. Nope? Find out, if they're on a fan fiction or role play site, what their character's name is. Their name is probably your answer. You get the idea.
Well you're at it, make sure the Scarlet Pimpernel really is their favorite book/movie/musical. Could be they're actually an A Tale of Two Cities fan, and that they only bought into the Scarlet Pimpernel in hopes of him rescuing Sydney Carton. If that's the case, I'm no help to you. I'll going back to my hacking of unsuspecting League members.