Fun character class! Some general comments:
- I’d replace with Alertness with Combat Reflexes (+1 initiative and +1 v. surprise)
- It feels like there should be a Graceful Fighting class power in there somewhere.
As far as the 5th level power, let me see if I’m understanding correctly. You want to allow the Kensai to enchant his magic weapon, following the rules for magic research in all respects, except that instead of special components being acquired and expended, the kensai must personally defeat the creatures.
I think this is reasonably balanced. But I think you need to be explicit a bit as to how it works, and give the power a specific name. I went with “anointed blade”. “Upon reaching the 5th level, the Kensai learns the secret art of anointing his weapon. An anointed weapon is sharper, lighter, and keener than any normal blade, and may have special powers against the creatures against whom it is anointed. [cut and paste relevant text, like we did for Machinist’s powers, then specify how the killing-the-foe function works.]” Then you need to explain some special cases - what happens if an anointed blade is lost? can an anointed blade be used by another character? can a kensai have multiple anointed blades? etc.