Looking at the kensei ability Path of the Kensei (Unearthed Arcana: Monk):
If you make an unarmed strike as part of the Attack action on your turn and are holding a kensei weapon, you can use that weapon to defend yourself. You gain a +2 bonus to AC until the start of your next turn while you are not incapacitated and the weapon is in your hand.
Does this mean a kensei with the Extra Attack ability can take an Attack action, attack once with a kensei weapon and then once unarmed and gain the +2 AC bonus for holding a kensei weapon?
Or have I missed something?
(I am stating that the Kensei is playtest material and not official 5e rules so that does not need to be a part of any answer)
(Just a note to say the rules mean the monk can't make a bonus unarmed attack because they are holding a non-monk weapon (even though it is a kensei weapon), but they can flurry of blows as that just requires an Attack option and has no weapon limitations)
Best Answer
Crawford and Mearls have commented that the UA Monk has some odd things in it with regard to the Kensei weapons. So, expect this to change when if/when they start considering it for inclusion. UA material hasn't been looked at for purposes like multiclassing; but for now:
Means, yes, if one of your attacks in your attack action is an unarmed strike, regardless of what the other was, you gain +2 to AC if you holding a Kensei weapon.