Hey Crimson, would you consider writing this guy as an NPC encounter instead of a companion? All the hard 'no's above can be pretty discouraging, but I'm a big fan of un-corrupting baddies (or at least swaying antagonists to the character's aims while still allowing them to keep their core values) so I'd be interested.
I think to make this a good NPC, you would want to revisit the concept to add more depth. See more steps between his active opposition to his interest in actively helping (and therefore betraying Kasyrra.) What hard choices would he have to make, and why would he choose to make those sacrifices?
Regardless, be sure to start him off as a one-shot NPC encounter so that if you find that either you don't have the time or the money for 50K, you can have him in the game as a story seed that you could take advantage of later. This way you can also polish the ideas and writing to fit into a reasonable amount of writing. Trim the fat, and all that. It's a major re-adjustment of expectations, and you wouldn't be able to squeeze all of what you want into one scene. However, writing contained scenes will be easier than a full-blown companion, and may still get you what you want in the end. (Eg: Instead of following the companion, he could set up some kind of base out in the wilderness where he houses a growing faction of rebels... maybe he has to be visited to avoid becoming a 3rd party nuisance that hopes to see both you AND Kasyrra ruin one another so he can win the aftermath? There's lots of directions that could go and a lot that you could write without keeping him by the player's side!)