So I've seen this theory get tossed around on the internet, so I thought I should talk about it. Basically the theory posits that Leni actually is diagnosed with autism. This would seemingly explain most of her behavior throughout the series. Going into it, she does meet some of the criteria: she has difficulty paying attention - even to something that is simplistic - and repetitive movements. Several fans had pointed out how Leni holding her hands in front of her body is one of the signature traits of a child with autism. Now while she seemingly meets most of the criteria listed, I honestly don't think she has autism, nor was it the creator's intent.
Autism is a neurodevelopmental disorder that makes social interactions difficult. In my opinion, Leni fails this part of the criteria. While she may not be that bright, Leni doesn't express much issue with forming social relationships. Granted, most of her friends share her interests so maybe that makes it more easy, but beside that, she