Okay this is a popular one but lemme just put together a list of reasons based on the Death Note Anime because this headcanon is so fricking exciting to me!! Feel free to add more.
-Speaks in a monotone voice
-Ignores common social rules of interaction, talks in a very blunt manner (“Matsuda, YOU ABSOLUTE IDIOT!!!”). Not necessarily obscene but sort of macabre language?
-On occasion, extremely good at empathizing, hence he can often be several steps ahead of Kira.
-Strong morals and sense of justice. Well, questionable morals at times, but strong nonetheless.
-Literally only wears one outfit consisting of a baggy shirt & jeans.
-NEVER WEARS SOCKS. Shoes only when absolutely necessary.
-The way he sits, with his legs up. Iconic. He says it helps him think. (Sitting a certain way for specific sensory input? Absolutely a thing.)
-Holds things “weirdly” (phone, paper). Dyspraxia?
-Can only eat certain foods. Literally. The anime only shows him eat sweets, snacks, and the occasional strawberry or banana. Drinks basically only tea with loads of sugar. Probably can’t handle savoury or spicy foods at all?
-Possibly has joint hypermobility or similar collagen defects (which are common-ish in autistic people). Could be the cause of his extremely pale skin, the permanent dark circles under his eye, bad posture and also another reason for him to sit his usual way.
- 24/7 fixated on the Kira case.
-Special Interests in crime solving (he solves cases on an international level under multiple pseudos!!) and especially the Kira case.