He reappears as a voice in Rand's head. While it is not clear if the voice is real or part of Rand's growing madness, it is treated as a real character.
He is tall with dark eyes, and brown hair turning white. His face is lined with worry. He wears gray, scarlet, and gold with the old Aes Sedai symbol on his cloak. (TEotW,Prologue)
During the Shadow War, he formulates a plan to seal the Bore. The original plan was to use a circle of thirteen men and women, but the women Aes Sedai led by Latra Posae oppose the plan. Instead, he and 113 men go to Shayol Ghul to seal the Bore by themselves. They succeed, but are driven mad by the Dark One's counterstroke. (TSR,Ch26, SaSG)
While Egwene chews out Rand, a voice in his head comments that Ilyena would never give him the rough side of her tongue because she was mad at herself. (TFoH,Ch2)
After Rand practices the sword and stops Bashere's dagger, Lews Therin starts talking. (LoC,Ch1)
When Rand is brought into the courtyard, Lews Therin rises in a frenzy in Rand's head. After some talking, Lews Therin raves "Kill him!" Mazrim Taim presents a seal and Lews Therin says "break it..." (LoC,Ch2)
When Rand holds saidin, Lews Therin rages "Kill him. Kill him." Rand shouts at him to shut up and the voice disappears for some time. (LoC,Ch3)
Some time later that day Lews Therin still has not returned. (LoC,Ch4)
Lews Therin reacts with confusion then stunned shock when Alanna bonds Rand. (LoC,Ch10)
Lews Therin laughs at Rand as he tries to run from Alanna's bond. (LoC,Ch11)
Lews Therin hums and thumbs his earlobe when he sees pretty women. When Cadsuane asks Rand if he's hearing voices and tells him it is a symptom of madness Lews Therin vanishes. (ACoS,Ch18)
Lews Therin is still gone and has not returned. (ACoS,Ch33)
After the explosion Lews Therin weeps that he should have sent Ilyena away. (TPoD,Ch29)
As Rand thinks of the inventions, Lews Therin murmurs, "I thought I could build. I was wrong. We are not builders, Not you, or I, or the other one. We are destroyers." (WH,Prologue)
Lews Therin hates being in Far Madding. He says, "How many will we three kill before the end, I wonder."(WH,Ch22)
Lews Therin sounds nearly sane when he says, "A man who trusts everyone is a fool and a man who trusts no one is a fool. We are all fools, if we live long enough." (WH,Ch25)
While Rand eats, Lews Therin comments that he used to like meat pies. (WH,Ch33)
Lews Therin howls when he finds himself caged again. (WH,Ch34)
Lews Therin sighs with relief when Rand leaves Far Madding. When Rand says he will use the Choedan Kal, Lews Therin shrieks that they were never tested. When Rand mentions Shadar Logoth, Lews Therin screams and flees. (WH,Ch35)
Lews Therin continues to mutter, but since Shadar Logoth he seems a bit more sane. (CoT,Ch24)
Lews Therin sees the image of Moridin in Rand's head. It is not one of the Forsaken as he knows all their faces. (KoD,Ch18)
When the Trollocs attack, Lews Therin seizes control of saidin ahead of Rand. He channels battle weaves from the Age of Legends. Afterwards, he refuses to release saying he wants to die. Rand convinces him to live until the Last Battle. (KoD,Ch19)
Lews Therin cries that he cannot continue the list because they do not know the names of the women who died in Natrin's Barrow. (TGS,Ch42)
Lews Therin thinks he likes Weiramon because he is loyal and honest. (TGS,Ch42)
In the Stone of Tear, Lews Therin recognizes the place where Rand tried to bring a dead girl back to life. (TGS,Ch47)
Lews Therin says death will be a mercy and he sounds a lot like Rand. (TGS,Ch49)
Lews Therin sounds completely sane when he says the reason for struggle against the Dark One is so they can have a second chance to love again. (TGS,Ch50)
ACoS,Ch2 - Perrin notices Rand staring off and muttering to himself indicating conversations with Lews Therin.
ACoS,Ch6 - Rand continues to have frequent blank periods indicating conversations with Lews Therin.
ACoS,Ch7 - From Lews Therin Rand knows that Sammael was at his best defending.
ACoS,Ch33 - Rand interprets Min's viewing of him and someone else merging, one lives and one dies, to mean that Lews Therin is, indeed, real. This cheers him as he thinks it is less likely that he is mad.
TGS,Ch15 - Min thinks her viewing of two men merging into one proves that Lews Therin is real.
TGS,Ch31 - Rand looks forward to dying as much as Lews Therin does.
TGS,Ch50 - When Rand realizes why he should fight on, Lews Therin disappears and Rand realizes he is gone forever and was never really a different person.
ToM,Ch3 - Rand tells Egwene that he has all of Lews Therin-s memories like a dream.
ToM,Ch5- Graendal partially succeeds in convincing Moridin that the attack on Natrin's Barrow was a -success- in that all those innocent deaths would have caused Lews Therin great anguish.
ToM,Ch51 - Rand tells Min that he has all of Lews Therin-s memories but there is a difference. He has her and all the others who love him.