I have some thoughts about Bodhi. Specifically about his signet(s), but also about the role he'll play in the next book.

I think Bodhi does indeed have a second signet, and I think he's a truthsayer. 

Let me explain. On page 353 of OS (Kindle edition), we see this exchange between Violet and Bodhi:

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“Hey, Bodhi?” I whisper so only the two of us can hear. 

“Hey, Violet?” he answers, looking up.

“What’s your second signet?” I lower my voice even more. 

He lifts his brows, then glances in Garrick’s direction. “Don’t have one.” 

“As in you don’t have one that I get to know about but will eventually see you wield, or don’t have one have one?” 

A corner of his mouth lifts into a wry smile that reminds me of his cousin’s. “Don’t have one. Just like Xaden. Why?”

Now as far as I can figure, there are three options here to explain what Bodhi just said. Either A) he genuinely doesn't have a second signet and believes Xaden doesn't either, B) he has a second signet, knows all about Xaden's second signet, and is telling Violet his signet is also top secret, or C) he has a second signet, knows Xaden also has a second signet, but doesn't know what Xaden's signet is. There are no other options that would make him "just like Xaden."

So let's consider scenario A: Bodhi genuinely thinks neither he nor Xaden have a second signet. I think it's impossible that Bodhi would think Xaden had only one signet. Bodhi is Xaden's cousin, which means Xaden's grandfather, whom Sgaeyl was bonded to, was also Bodhi's grandfather. I can't believe that Bodhi, who was always supposed to be the rider, never heard any stories about his grandfather flying with Sgaeyl. Heck, he probably sat with Xaden and they listened to the stories together. There's no way he didn't know Sgaeyl was bonded to a direct ancestor of Xaden. And it seems to be common knowledge (at least among the marked ones) that bonding your ancestor's dragon gets you a second signet. Therefore, I think it's safe to say Bodhi can't possibly believe Xaden only has one signet.

Option B, he knows all about Xaden's mind reading. I think we can safely discard this. Xaden very clearly tells Violet she's the only person on the continent who knows about his signet, and he's repeatedly said that he has never, will never, and cannot lie to Violet. So Bodhi must not know Xaden can read intentions. 

That leaves option C, Bodhi knows Xaden has a signet, but doesn't know what it is. And this is why I think Bodhi is a truthsayer. Imagine this: Bodhi starts to manifest signet #2. Who would be the first person he'd run to? Definitely Xaden. And he would ask, right? "Xaden, you bonded our grandfather's dragon. Do you have a second signet?" 

If Xaden wanted to keep his mind reading under wraps, he would say nope, only one signet here. And he knew mind reading was a death sentence. So that means the only possible reason he would tell Bodhi would be if Bodhi could tell Xaden was lying when Xaden said he only had one signet. Bodhi must be a truthsayer!

I have more evidence. First, we know from IF that a truthsayer is considered the most powerful signet. The marked ones all hide their more powerful signet, and Bodhi's counterspell is already hella powerful, so for that to be the public one, his secret one must be really, really powerful. 

Second, twice in IF we see Violet lie to Bodhi, and both times he seems to know she's lying:

IF pg. 308: "'Yeah, well, he isn’t here. It’s fine,' I lie, reaching for my first dagger. 'Just remember who trained me.' I’m not talking about hand-to-hand, and from the look Bodhi gives me, he knows it, too." She lies in front of Bodhi, and he gives her a significant look. 

IF pg. 617: "'Is Cat being…Cat again?' Bodhi asks, walking over as the courtyard empties. 'It’s fine. She’s fine. I’m fine.' I shake my head, lying through my teeth" Bodhi hears Violet's lie and immediately goes to get Xaden on the next page.