The reason I changed from the standard archetype and had it start at 1st level was because of the single-weapon focus (and I used the original Oriental Adventures kensei as a starting point). I thought the flavor of the class would warrant that this character wouldn't bother with other types of weapons - he's a true specialist. If I waited until 3rd level, then this flavor gets muddied, but I did see it as a fighter-base so didn't think it needed to be a brand new class.
I think I need to add that this, like UA material, is not balanced with multi-classing. My groups don't use multi-classing. I should probably make that explicit in the design.
Obviously this class is meant to be a glass cannon - the single weapon weapon is definitely more powerful than other weapons of its type, but in focusing on this weapon the kensei gives up utility: he is basically either melee or ranged, he is limited to one type of damage, he doesn't use a shield, he doesn't use heavy armor, he won't use a weapon (magical or not) not of his weapon type, etc. That's also why he gets an additional feature at 1st and 2nd levels.
Otherwise, as a fighter, he'd have two levels with drawbacks with no additional benefits.
Here is an update with some re-working based on the helpful suggestions above:
- restricted from multi-classing
- removed attack bonus at level 2
- changed "double proficiency bonus" to "add proficiency bonus" for initiative at level 2
- removed the critical hit bonus at level 9
- removed the disarm ability at level 10
- removed GWM/SS feature from level 13
- substituted Indomitable (at standard fighter levels) for existing features
