First of all, I don't troll or lie or do drugs.
Anyhow, as for the new chara, there's always a possibility that she's just another undercover Kafuka. On the other and it's about time for new characters and this girl just doesn't look lie Kafka at all. She reminds me of Chiri somehow and since Chiri has sister and it was kind of underlined that it's somehow meaningful for the future events, this might as well be it. And by the way, it wasn't exactly stated that she was a junkie and isn't that onion based health injection kind of popular in japan that can also be referred as "injecting juice" or something like that. Anyhow, the second fact that underlines the possibility that she indeed is Chiri´s sis is the way how Chiri reacts that gets more attention than the other characters reaction. I mean her panel is way bigger and sinister. As for compulsive-lying, if the note she hands to sensei is true, it wouldn't really make no sense since we already have our very own romantic stalker girl. So I'd say that the girl is pulling a bluff and it has to mean something since this is her introduction chapter. So therefore, I'd say she's a compulsive liar.