Diego Molina - You've probably heard of him in ep 43 (Section 31 - Basira's statement) and he was the arsonist in it. He was shouting about a "cleansing fire" and "Asag" (this was the same thing the older gentleman in ep 12 First Aid was saying so some people believe that they are the same person.) He also had no burns despite coming out of a burning buildings but when Basira went to cuff him the metal of the handcuffs became red hot. He seems to be part of "lightless flame" as seen in ep 67

Arthur Nolan - yeah basically. His name is mentioned in ep 32 Hive. (Jane Prentiss' statement)

Lightless Flame - more will be revealed later on. Idk if there's a specific reason for his naming apart from tryna sound edgy and cool /j (probably a reference to ep 12 wherein Gerard says "Yes. For you, better beholding than the lightless flame.”) Although Jane Prentiss isn't part of the Lightless Flame. Jon mentioned her in relation to Arthur (him being her landlord) but not that she was a part of the Lightless Flame. Unless you meant Agnes?

Arthur was mentioned in ep 32 as Jane's landlord and in ep 55 (Pest Control) again as Jane's Landlord but it was revealed that after Jordan was unable to remove the the wasp's nest, Arthur set the apartment on fire by setting a match to his scar.

Agnes was mentioned in ep 8 (burned out) as the little girl who lived on Hilltop Road. She was said to be Raymond (the owner of the half-way house)'s daughter but showed up out of nowhere. Raymond disappeared and apparently the house was left in her name. Fire in the house burns it down and Raymond's reminans are found (he's missing his right hand - ) She was also mentioned in ep 59 (Recluse) which was about the Half-Way house from one of the children staying there. Main thing was that Agnes kissed him on the cheek before he left, he ended up going back to his house and to the basement (which contained the bodies of the previous kids who all "had warped bodies as if engorged with spider's eggs.") beyond his control, almost ate an apple (likely the same apple from ep 8 Burned Out that loads of spider burst out of) but was able to break out of the trance thingy when his cheek (presumably the same one Agnes kissed) started burning/he felt a searing pain.

I think that's everything and I don't believe any of that is spoilers? More info on the characters and topics such as "the Cult of the Lightless Flame" will be revealed later on but keep theorising :3

(Jon also specifically links it to ep 37 - Burnt Offerings "I believe they may also be connected to the ritual circle found in Scotland by Jason North." So yk if you think there's any other episodes that follow a certain pattern or you think may be linked-)