L has DID (pt. 1, overall analysis)
Pretty much since the first moment I began thinking about L more deeply, I've read him as having Dissociative Identity Disorder.
The main reason I think he does is because of vibes and "it just feels right", but I feel like there's a lot of supporting evidence for it, too.
Quick disclaimer that-
A lot of what I'm going to mention can also be explained by other things (e.g. loss of awareness for bodily needs is a common symptom of autism), and don't necessarily point to DID on their own.
Systems have a wide array of experiences, and not everything I'm going to bring up will be relatable or applicable to every system.
With that said!
Here's a rundown of the main things that support this theory:
We don't know much about his life before Wammy's House, but seeing as he was an 8-year-old orphan with no name, I'm sure it was rough. He was also well-versed in self-defense enough to fight off multiple older kids who were trying to harass him. It's likely that a child with this kind of background may have developed structural dissociation patterns to survive, especially if their brain were predisposed to doing so.
The fact that he was 8 years old with "no name" is very interesting in and of itself. There are three possibilities here.
One is that he genuinely was never named anything, even temporarily. If he experienced neglect to that degree, it would make it very hard to form any stable sense of identity, and be a very traumatic situation that would force him to develop extreme self-reliance.
Another possibility is that he's been temporarily called different names throughout his life, which could in and of itself contribute to a fracturing of identity, when paired with whatever trauma he was experiencing that lead to this happening.
The third is that he used to have a name, but for whatever reason, either forgot it (since L seems to be the type to have an extraordinary memory even from a really young age, this could easily point to dissociative amnesia due to trauma, unless he was a baby when he was separated), or disidentified with it to the point where he no longer considers it his name at all.
L seems to have a very strong disconnect with his body. He has poor posture, sits in a folded-up position for hours upon hours without moving, eats only to fuel his brain enough to focus, has a very unnatural sleep pattern, and so on. He doesn't seem to notice or care about whatever physical pain and discomfort his lack of self-care would bring, instead prioritizing whatever helps him focus and gives him a sense of personal autonomy.
I'm going to highlight this part of it - he's disconnected from his body in favor of the analytical parts of his brain. That's an extremely common dissociative experience, to be perpetually in the state of the observer. While I think he does have strong emotions, and just doesn't express them the same way allistic people might, I think that he (the observer) rarely steps into those emotions fully. He notices them in his body as if they're a separate entity, and logically considers their cause and what to do about them, as he does with everything he encounters in his external reality as well. He's locked in this alert, watchful state where he can feel in control no matter what.
There are a lot of aspects of his job which require and/or would greatly benefit from very heavy compartmentalization. If he works well with his system, I can see how his DID could help him excel with that, and I can even see him being drawn to this career partially because the work requires patterns of behavior he's already very versed in.
He definitely has PTSD. Not only would it be practically impossible to have his kind of childhood, and work in his kind of career, without developing PTSD, but he shows a lot of symptoms. (Heavy distrust of people, isolation, detachment from his body and emotions, high need for control and certainty, tunnel visioning on a threat until he feels completely safe from it, very alert, extremely cautious, constantly folded into self-protective postures, prone to depression, etc. Also experiences hallucinations when triggered, very stressed, or feeling utterly helpless, depending on how you interpret the "the bells are very loud today" scene.)
His PTSD seems to be very dissociative in nature. Most of the time, when he's in a dangerous or upsetting situation, he's able to quickly emotionally detach from the situation. Even on the rare occasion where he's pushed to a big obvious emotional reaction, he's able to quickly regain his composure and switch back into his cool, collected, logical mode. In fact, this manner of dissociation seems to be his primary reaction to feeling threatened.
There were several points in the anime that felt like other alters were present. The most obvious was when he had that huge reaction to learning of the existence of shinigami (and his quick switch back to his prior mannerisms), but there are also distinct fluctuations in his abilities, mannerisms, perspectives, priorities, and so forth in other moments, such as the yotsuba arc or the morning of his death. While these differences can be explained by other things, I think they could also be a sign of alter activity.
Mello's narration in the LABB novel is very DID-core:
I have personal headcanons on details of how L's system works, but I'll contain those in another post, since they're a bit more subjective.