This character has autism.

Autism is a disorder of neural development characterized by impaired social interaction, by impaired verbal and non-verbal communication, and by restricted, repetitive or stereotyped behavior. The diagnostic criteria require that symptoms become apparent before a child is three years old.

Only use this tag for characters who are explicitly said to be autistic (within the story or by the creators). Do not use it for characters who are simply too shy to properly communicate, for example.