how will the secret that Mon el is hiding from Kara effect their relationship once she found's out

My assumption is, that Mon El is the Daxamite Prince who used his privileged status to escape the planet's destruction...and once Kara finds out feelings or no feelings she's gonna clout him a good one.
Because that....cuts across her survivor's guilt and flies in the face of her egalitarian impulses. I'm fine with the romance and I'll be fine with the retribution as well...

JF

I do not think that Mon-El will fare well with Kara. First of all because he lied to her by omission. Second, the question of how he survived. This is despite the fact that his guard tricked into the pod.

I don't think I'd want to be him when Kara boils'!

Argo

I don't know about taking over the world, I don't think Daxam has the resources it used to. Besides Kryptonians already did that last season.

Judging by the way Kara reacted to James not telling her he was The Guardian, I'd say its a safe bet she will be pissed when she learns that Mon El is the Prince

On the other hand Mon El's parents aren't likely to a approve of a Kryptonian dating there son, what with the Krypton/Daxam feud and everything. Plus we know they have arranged marriages on Daxam so as far as his parents are concerned Mon El is already spoken for.

I doubt they will be quiet about it. They will probably insist that Kara and Mon El break up.

So I can see Kara being really angry at Mon El, while at the same time not wanting to give his parents the satisfaction of messing up their relationship.

I predict that the training area will soon be covered in gravel.
Kara's going to need more cinderblocks.