how much of L do you think is an act? i know the author's intent is L-always-lies. His design is very different before he meets the task force and after. do you take this as simple design development or L putting on a 'weirder' personality to distract? do you think he ~likes the people he works with, even in a detached way, or they're simply a means to an end? do you think he could like light had he not been a criminal or do you think he genuinely despises/doesn't care for him in any context?(1)
Hi anon, thank you for the thoughtful ask! Those are definitely the questions to ask about L, and about L and Light.
I personally think L is not putting on a persona or an act in any way once he meets up with the task force, and that he's very often being pretty bluntly honest about himself with others as well. His design and his mannerisms changed somewhat between his first chapters in the manga and the time he met up with the task force simply because Ohba and Obata hadn't fully decided on them yet, and they also wanted to reveal some of his eccentricities like his sweet tooth more gradually over time. They explained all this in detail in the behind the scenes book. And I very much prefer to think of it simply as L's design changing, because once you really get going with the "persona theory" hypotheticals it is usually nonsensical at best, and cringey and offensive at worst.
So I think that L basically only outright lies when it comes to trying to trip Light up with the investigation somehow, such as when he pretends that there was another fake note mixed in amongst the ones Light had the prisoners write. I think that in the rare moments that L does reveal concrete information about himself, like when he tells Light that he lived in England for a few years in the past, that he is very likely telling Light something that is true (because he is super calculating about what he reveals, so he already knows it will be fine to reveal, and also very competitive; he would probably rather tell Light something real to bait him or taunt him than he would just make up some random lie). And whenever L tells others things about his personality, like that he is competitive and childish, I think that he genuinely sees himself that way. He isn't trying to deceive anybody in those instances, but is instead trying to be pretty upfront about himself.
I think that most of the deceptiveness people sense in L's character just comes from how immensely difficult he is to read in terms of his true motivations and emotions, and how continuously mysterious as a person he is to Light and to everybody else. I'd say it isn't so much that L is deviously putting up all these smoke-and-mirrors façades around others as that he is just proud, emotionally guarded, and notoriously difficult to get to know personally. And I think he is that way both intentionally and unintentionally sometimes. I think he's that elusive as a person both in order to protect himself during the Kira investigation, and also because he is a bit snobby and prickly and awkward; he probably doesn't actually have that much interest in getting close to other people most of the time. I think he usually only really cares about doing his job and being right and having fun.
I also think that L likes to be slippery and sarcastic sometimes in order just to tease people, and that he somewhat downplays his own stubbornness and self-awareness occasionally to slyly get away with things he knows that other people might not agree with him doing during the investigation, if that makes sense? But he probably also hides impressive and considerate things about himself sometimes in a humble sort of way as well; like when he had secretly set up life insurance for the task force and their families, and acted a bit weird about it being unexpectedly revealed by Watari in front of everybody else.
I don't think that Light despised L during the Yotsuba arc, but I wouldn't have blamed him if he did. At that point Light was being held prisoner by L and genuinely thought himself innocent, and knew that he would likely be put to death by L if he was actually found to be guilty for his crimes. But then the more he tried to help L and prove his innocence and win his freedom the more suspicious L still found him anyway. Lol. Light had already been put through a long confinement and traumatic fake execution at the hands of L and his own father, and then L handcuffed him to his wrist without consent and proceeded to sulk and mope around about the investigation for weeks and weeks on end. Light barely ever thinks anything negative about L in general, even during the Yotsuba arc, but I find that to definitely be L's least sympathetic arc in the story in terms of the shady and controlling behaviour he pulls towards his coworkers and his suspects. So I wouldn't really have blamed Light for disliking L at all when he didn't have his Kira memories intact.
But that being said, I don't think that L and Light just straight-up hated each other at all. I think it was a lot more complicated than that for them both. I think they really enjoyed competing with each other, and that they related to each other in a way neither of them had probably ever felt before because of their lifetimes of feeling bored and superior and aloof and talented and set apart by other people around them constantly. I think that Light definitely showed in canon that he never got over L in his own mind, and I think that it would have been similar for L about Light. Though for some reason I feel as though L would maybe be a bit less likely to admit this to himself or to other people out loud.