Well, the facts are

1. It is somebody who was willing and able to commit a murder to frame Lottie
2. Somebody who (frequently) appears and resembles Lottie quite a bit
3. Somebody who is stepping it up – the appearance in the coffee shop was bould to reveal to Lottie that there is an evil doppelgänger/foe/entity out to get her

All those villans from the past don´t fit the bill.

My bet is on

a) a girl from school/Tackleford/Wenford she has mortally offended
b) Some unknown detective jealous on Lottie´s success (The assisstant of the murdered Miyamoto deliberately mistranslated to put Lottie in bad repute with him)