To OP.
Ninjas use Katanas only for RP use. So many swords are better all-round than katanas..
The Danish and German greatswords are in my opinion the very best ones for all round use. Perhaps a bit longish for some siege situations but nevertheless. The range make it easy for them to stay far away out of 1h ranges, and they do way more damage than Katanas..
You should also know that most Ninjas focus on 1 thing build wise. Speed. Swing speed and athletics + low armor gives us the kind of armor many other players don't have. My own build is probably the fastest practical build out there, and coupled with the fastest sword it is quite mean.
Athletics=Armor. You can run away from any engagement, and almost no one can catch you. You can choose your battles. Kite people. Create situations where you have the advantage, pull the enemy into disadvantage, control the fight, jump places other players cannot, surprise-jump cav, be places faster than people would think you could be there, the list goes on. Only similar 8-9 athletics builds gives you problems.. Archers miss you because they either:
1. Have problems leading you because of unconventional movement speed.
2. You can dodge arrows more effectively.
Cavalry has trouble because you can either dodge them, close on them too fast, jump-spear, jump slash..
Of course, the trade off is that there is little room for mistake, as you go down in very few hits, you don't do much damage, you usually must block the first hit from other polearms/2h to get close enough to deal damage.. Many of the popular 1 handers actually outrange the katanas now, and some of them approach katanas in speed. (But that's how we like it)
With only 4PS I absolutely depend on hitting the heads of other people to survive to have efficiency. Perhaps you met some specialist Ninjas that did insane damage, well that was the reason. Tin can's can go down in 2 or 3 well placed hits though. The average hits to take down most players is 2-3 for me..
My build:
Level 30 (4 420 577 xp)
Hit points: 49
Strength: 12
Agility: 27
Converted: 8
Ironflesh: 1
Power Strike: 4
Athletics: 9
Weapon Master: 9
Two Handed: 181