I'd like to explore Ronan's relationship with truth and lies. In TDT, Ronan says he never lies. (chpts 21 & 44). In TRK, Gansey tells Adam that Ronan always tells the truth. (chpt 14)
(Spoilers)
But he does lie. Maybe not habitually, like Declan, but as much as most people.
He lies through action when he dreamed the evidence to frame Colin Greenmantle with crimes unimaginable. (BLLB chpt 31) as revenge for Niall's murder. He lies by omission about Mathew's status as a dream. (CDTH chpt 52) He lies to Adam about having never thought about making a dream copy of Aurora, because he wants the conversation to be over (MI chpt 19). And Ronan lies when he goes to visit Aurora on the ley line, and she asks after Declan.
" 'And how about Declan?' Aurora asked. There was a pause, just a breath too long. "Working," Ronan lied. Everyone in the Rose Glen looked at Ronan." (TRK chpt 31)
The funny thing is this might be factually true. Declan is now in DC as a political intern, and probably is working. Aurora's response ("He was always so diligent") indicates she finds it plausible. The real lie is the implication that Ronan checked to see if Declan was available and wanted to go visit Aurora. Judging by his friends' reaction, he didn't and they all know it.
So why does Ronan tell himself he never lies? And why is it important to him to think this?
First, Ronan and Gansey have both confused honesty with bluntness. Bluntness is a very specific type of honesty in which you tell others about how you feel about something or someone, regardless of courtesy or kindness. (For Gansey this may have to do with his upbringing. Gansey was raised by people who did not speak directly in order to avoid unpleasantness, so Ronan's bluntness might have been a relief. At least he knows where he stands with Ronan.)
Next, it may be to distance himself from his family. Declan, for whom he has no respect, is a habitual liar. And Ronan is becoming uncomfortably aware that the life Niall had built for them was based on dreams, secrets and lies. This obviously troubles him, because he dreams
"It's a loophole," his father said. "A loophole for thieves."
"Is that a lie?" Ronan asked.
Because Niall Lynch was the biggest liar of all and he'd stuffed that all into his eldest son. There was not much difference between a lie and a secret.
"I never lie to you." (TDT chpt 52)
Finally, Ronan is a very powerful Dreamer with the ability to rewrite reality. Opal says,
"she was to tell the truth, because Ronan always told the truth. This was the most unfair rule of all, because Ronan could dream himself a new truth." (Opal)
You see this when he dreams a new Camaro into existence, which Gansey accepts without questioning, even though Ronan told him the truth about crashing the Pig. You also see it with the new, dreamed will, which Declan oddly never questions.
Later, when he dreams the portrait of the Dark Lady in front of Hennessey's girls, Jordan notes
"It broke Jordan's brain. It wasn't that he woke and things appeared suddenly beside him. It wasn't that they faded into existence. It wasn't anything that easy. It was more that he woke, and something about the time around him changed, something about the way everybody experienced time . . . Somehow reality had been edited to allow for the presence of something that hadn't been there before, without allowing her the reality of seeing it come into being." (CDTH chpt 48)
What a frightening ability this must be for Ronan. In MI, it's revealed that Ronan's worst dream is the inability to distinguish dreams from reality. Ronan must to cling to the idea of his own truthfulness, to assure himself that he would never confuse truth and reality within his dreams. Otherwise nothing in his life can be trusted.
But in the end, Ronan is lying to himself about not lying and he creates Bryde, and the rumors surrounding him, without realizing it. Bryde seems designed both to tell Ronan all he wants to hear, and to gaslight him into believing it. And consequently, the Dream becomes stronger than the Dreamer, and Bryde remains awake, while Ronan continues to sleep.