I'm actually really loving the fact that he didn't believe her head on, it would be totall...

i think it's ooc that he didn't believe her - not even a little bit - since he is (was?) completely in love with ellin. imagine your partner not believing in you and possibliy thinking she's a deranged lunatic even though you are about to get married. yikes. also he's a smart character - are you going to tell me he forgot about the weird things MC would do in beginning of story at the library re:languages and writing?

He thought she was a spy and probably found a way to justify all of that. Imagine your lover saying suspicious things in a very critical point of your life in which you lose a very old friend, and then it seems like your lover plotted to kill either your or your friend. When your friend is dead. It's easy to say things like this when you know everything that happened, but he didn't get the ability to experience things from her side.
Yes, mistrust is terrible. But it's a very reasonable response. And after how she acted, he has every right to feel this way. What she's saying is insane, he is not a kind of person that will believe it straight up.

I don't think so. I'd believe my partner first because I wouldn't be with that person if I had doubts of their sincerity in their first place. it is so offensive that he thought she was outright lying. even more offensive that he feels he needs to cover for her. so if you (as in kiha) don't fucking believe her and suspect her for being a traitor, then what's keeping you tethered to her and the relationship? sex? pathetic, really.
and besides, why would he assume that he someone allegedly respects would give him such drivel if she didn't mean it? dude is out of pocket. if i were ellin, i'd rethink my relationship with this man. she must feel so lonely as the person she loved the most in this world didn't believe her.

fixing this sentence to make it clearer: [...] why would he assume that someone he allegedly respects would tell him all that if she didn't mean it?

First they are only starting out, he didn't know her that long. Yes he loves her but they knew each other for like, 2 months? 3? It's just the beginning. It's not like he knows her for years and can tell in a heartbeat just what she feels and thinks. Also, what she is saying is really weird. it's weird in our age with awareness to mental health etc, so then? Even today, this is a lot to take in and understand. He has the right to be confused, scared and suspicious.

Also, it's a different time. Everything was used as ammo at that time - woman, children, whatever. I think you might be looking at this with a 21 century perspective of trust your partner and so and so, when back then if you where high up running a country, things simply worked differently. We just saw an adoptive mother murdering her son, trust has no factor here. Most of the countries are sheltered and fortunate to live like this nowadays. Back then, they really had no protection.
Also, Ellin acted in a very suspicious way from his perspective. She told him that she never loved Jin, then she told him that she thinks heking light will die, king light dies and then Kiha discovers she had a diary filled with love towards Jin, in which she plots to kill him or king light. Of course he'll get suspicious. Kiha is a very smart sensible guy, that thinks with his head first and everything else second. If he wasn't suspicious and wholeheartedly believed Ellin when she told him, it would have totally betrayed his character.

He keeps her because he loves her, it's very clearly that (I'm pretty sure he said that so I'm not sure where this claim comes from). You can love someone and still feel misguided, confused and even betrayed. Sometimes relationships are not perfect.

you're telling me this "first they are only starting out, he didn't know her that long" as he considers covering for her betraying the king as a spy?!
the biggest issue here is that a relationship is built on trust and if you're convinced your partner is a raving lunatic + liar, then there's nothing to build upon, other than what? sex?

I'm starting to think this is a major plot issue (rather than just a kiha character deviance) bc this just doesn't make sense.

Now Kiha has no influence the country and on the throne, so he doesn't care very much if Ellin is a spy, he just care for her that much. The thing that bothered him (at least how it seems to me) is the fear of her being dishonest, especially in the beginning of their relationship (about her relationship with Jin etc), also the "fact" she wanted to kill him. Basically that's it. It was made pretty clear in my opinion.
Now she's only going to influence him, not the country. So I believe this is why Kiha has no problem being the one to fall alone. It makes little sense for him - that cares for her this much - to let the king know. When we literally see how much influence she holds over him over the story.
Kiha very clearly does not care about himself and he seeks people and goals outside of himself in order to be fulfilled and go on with his life. Ellin is pretty much everything he's got. I don't think he cares to take the fall if it's for her.

And honestly the argument of "what does he want to do with her? sex?" is just extremely flawed and misguided in my opinion. When something shocking happens in a relationship your first instinct is not to think "so what is the intention in what I'm doing", it's to keep going somehow. Whenever it's to break up, stay together or whatever in between, you don't think "oh I'll keep herhim for sex or whatever". You just act and think later. We got a scene just like this when Kiha barely knows what he's going to do and it's very clear he has little to no idea what he's doing.
If the problem was long term and things were bad, sure, I could get behind you. But he literally just found out. He's barely holding his thought together. He didn't even thik this through. Give the man a break PLEASE XDDD

your arguments make absolutely no sense.
like, does he care for her or NOT? if yes, does he believe in her or NOT?
if he does NOT believe in her, then what tf is his motivation to be with her?! would you be in a relationship with someone you don't believe in?!
this is not an "omg a little white fib" .. this is HUGE, and the fact that you keep chucking this major point aside just to try to prove your take is baffling to me.

lmao character lost all my sympathy when he brushed aside ellin as a liar/crazy person so nah i'm not going to give him a break bc this development is wack.

Trust and care are not the same things, and you can care for someone you don't trust, and trust someone you don't care for. It's not the same thing.
And again, he just found out. He barely knows what he's doing. He didn't even think this through, and he doesn't understand the possible outcome of what hes doing. He needs time and that make total sense. Also I never said this is not huge, I'm saying it's a lot to take in and that he's shocked so his response is reasonable, he is human.
He has no motivation but her, the guy just want her to be safe and is willing to do anything for her. So it make sense that he is making questionable choices, not sure what he's doing and is doubting her. It's neutral.