Becoming the Mask: L being friendly with Light was initially for the case but he came to genuinely care about him. Light genuinely cared about L during his Memory Gambit but he seemed to lose this when he regained his memories... but then it turned out he was lying to himself.
Bedlam House: L plans to send Light there. But "it has a very nice view."
Corner of Woe: L returns to HQ to find Light sitting in the dark (see Evil Is Not Well Lit below). It's the result of L and Light having a nastier fight than usual; culminating in L giving Light his own psych file to read.
Death Seeker: Light, possibly. After L breaks up with him he has been engaging in more and more self-destructive behaviors. L when he moves against Kira he confronts him alone suggesting they could die together here and then in a clearly suicidal move he gets between Mello and Light.
V: She was a victim of a human trafficking ring and had a child by rape when she was 12 (a child that would later be stolen from her by Wammy’s House.) L discovered the human trafficking ring and also how V had arranged their deaths, and so recruited her to be an assassin for Wammy's House. But because she is an assassin they wouldn't let her talk to her son or let him know she was his mother and still alive. Also there are hints that she might have been in love with A who committed suicide.
Near becomes suspicious when Kira immediately recognized that Near was a fake L and Mello notices that L and Kira seem quite friendly with each other when L is on the phone with Kira for a long time...
Functional Addict: After Kira has "won" Light becomes extremely bored and depressed. He has all these minions to take care of him and he doesn't know what to do with himself, he has too much time on his hands and too many memories he doesn't want... and he has access to a doctor who can get him anything...
Love Makes You Dumb: L and Light are at an impasse. They could both easily destroy each other but they don't. It's gotten to the point where the minions/underlings on both sides are beginning to wonder what's going on.
Love Makes You Evil: Light and L. Honestly neither of them was particularly nice to begin with but they really bring out the worst in each other.
Love Martyr: Both Light and L. Let's just say they have a really twisted relationship. Also Misa's unrequited love towards Light.
The Man Behind the Curtain: Kira has kept himself so secluded that many people are beginning to doubt that he actually exists as an individual and that everything is being done by the Kira supporters and Astraea.
Mello: You'll go down in the books, Matsuda. In the chapter called "Fucking Idiot Cowards of History", you'll be right at the top of the page. And L will be in second place after Liza Minnelli in the 'I Have Really Shit Taste in Men' awards.
Matt: I know you're Kira or whatever... this is me impressed. Doesn't change the fact that if you start any funny business with her, I'll shoot you through the fucking wall. I'm quicker than you and your little Diary of Death. OK?
Light: It was you, wasn't it? You fucking tried to kill him!
V: I'm sorry, Light.
Light: Not going to cut it, V. I thought that you didn't want to kill L.
V: I don't.
Light: If you touch him, I'll kill you. I'll kill all of you.
Light: I wonder what N stands for... 'Nobody', 'Notus', 'Nomad', 'Nincompoop', 'Norman', 'Nemo', 'Neo'... 'Norris', 'Nathaniel', 'Noah', 'Ned', 'Nando', 'Napoleon', 'Nelson', 'Nickelodeon', 'Nicodemus'...
L: Near... It stands for Near, Light. Remember it. I'm glad that you're showing an interest though, for I never expected any less of you. However, I am most insulted on his behalf that you think that he could be named after a lost fish or Keanu Reeves.
Light:L, when you've been who I've been, you can't exactly get a job in a service station.
Replacement Love Interest: There seems to be a bit of tension between Mello and Near. Also Light was considering screwing Mikami.
The Revolution Will Not Be Civilized: Astraea. Later when L begins La Résistance under Kira's rule Mello sees a simple solution to the Kira problem- he proposes that they find Kira and blow his head off. L notes that it's perfectly practical plan... but he has no intention of doing it.
The Runaway: Mello and Matt from Wammy's House. Light too, sort of, from L and his family. He's fleeing L having him committed but he has a place to go- he takes control of Astraea and then takes over the world.
Sanity Slippage: Light becomes more and more batshit as he's torn between his duties as Kira and his love and loyalty for L:
Light's heavy notebook usage and his new drug addiction isn't helping. Also L has this for a while too after Watari dies until he gets Bored With Insanity later. It's also revealed that L's "insanity" was being caused by Light drugging him.
Selective Obliviousness: At first Mello REFUSES to believe Near's theory—that L lied to him and that Light Yagami is Kira.
Serial-Killer Killer: Light and Misa of course and V. She was already damaged when Wammys house got a hold of her and decided what with her Sociopathic tendencies she was beyond saving so it would be more productive to just make her a more effective assassin.
Sex Face Turn: When Kira returns, Light is much more reluctant to kill L now that they've been sleeping together.
Sherlock Scan: Near deduces the following about Kira from speaking with Light on the phone:
Near: Kira, if it was Kira, sounded like a Japanese male in his late teens to early twenties... Slim build. He was probably wearing a suit since he was definitely wearing a tie. Irritable.
"...any therapy would likely further enable his abilities to manipulate others. His intelligence and superficial charm, combined with his pathologic egocentricity and internalised morality, make him a dangerous individual deserving of clinical management and incarceration. He is driven by some, as yet, undefined goal. Kira's targeting of criminals gives a certain mask of sanity and 'goodness' in terms of his reasoning, but, due to his narcissism and deep-rooted antipathy, I doubt that they are exclusively or will remain his true aim of attack. At this point in time I would score him high for the Factor 1 scale of emotional psychopathy on the PCL-R, though his neurosis are not fully incapsulated within those criteria. He appears, at times, to show feelings of strong empathy, despite his apparent incapability to love." Light slammed the folder and the words shut. "What a load of utter rubbish. I don't care. Really. I couldn't give a flying fuck."
Sociopathic Hero: After Light regains his Kira memories but still considers L to be his "partner" in destroying Astraea.
L: It must have quite distressing to come home and find your family murdered only to watch the perpetrator evade justice... However, considering that psychological trauma, she is most definitely and unsurprisingly unbalanced. She exhibits signs of erotomania in relation to you, Light-kun.
Light: I wouldn't go that far... It's a crush. I can handle it.
L: Yes, by avoidance. I dealt with a case where the victim handled it in the same way as you're doing. Didn't end well.
Light: They died?
L: No, but 56 other people did when their stalker decided that their deaths would be an appropriate tribute.
Sometime ago, Matsuda had left a post-it on one of the laptops in which he'd accidentally managed to wedge a biro into the CD drive in an attempt to extricate something. "Pen is stuck in drive" Only the 'pen' and 'is' were written too close together. L now had a 'penis stuck in drive' post-it stuck on his laptop screen. He was considering framing it.
L: Oh, my apologies, I forgot that you're the sexual scourge of womankind. You obviously only have sex with me because you roll over in your sleep and misjudge the distance. Maybe you're sleepwalking... or maybe the term, 'sleepfucking', would be more appropriate. Maybe you're completely unaware of what you're doing.
Light: It happens.
Trash of the Titans: L resorts to just throwing off the huge stacks of paperwork, trash, and finger puppets from the hotel's (taskforce HQ's) coffee table in order to find his lollipop.
Written by the Winners: Light upon meeting Matt and realizing, to his horror, that once he's secured his new world this would go down as the first meeting of Kira and Astraea-this is going to be in the history books!
Yandere: Misa towards Light and after Light regains his Kira memories he becomes both possessive and obsessive of L. He begins making up excuses to himself why he isn't going to kill him now like "it's not a good time," or "I want to break him first" (suggesting he really does love him underneath the psychosis. Light becomes very angry and upsetwhen someone else tries to kill L.) Light arranges things so that Misa kills Watari, and then he kills her because he felt threatened that Watari was going to make L break-up with him and that Misa (a Yandere herself would try and kill L so that she and Light could be together). Light keeps L drugged up for weeks afterwards and spends so much time with him that his infatuation becomes rather obvious to everyone at which point Light even considers killing his own father because he's against their relationship. Later he does use his father's death to further his schemes.
Yank the Dog's Chain: 1) Just when Mikami is thinking he's going to get to rule as Kira now he gets blown up. 2) Light and L are just thinking about settling down in a nice cottage in Scotland when Ryuk finds them again.
Yes-Man: Mikami. Light notes that his vocabulary is limited to "Yes, Kami."
Matt: He's always prissy, sitting at his desk like, 'I'm the king of you' to everyone and then Kira prances in and ol' Miki's literally throwing himself on the ground all, 'Take me up the arse, oh mighty one.' Bloody eejit.