Lioness
Be aware that if you dual-class (to mage, say) you'll lose all kensai abilities until your mage level is one better than the kensai. So if you d/c at lvl 9 kensai, you only get your kensai abilities back when your mage reaches lvl 10. So if you intend to do d/c it's best to do it early.
Whether the kensai is effective depends on the make-up of the party and developing a playing style that takes advantages of your party members' strengths while minimising their weaknesses with magical items or by strategy. From my experience the kensai can be an effective tank, the only limitation being armour and missile weapons (a kensai can't use both).
My kensai is level 20 (or 21) right now, and I've just about wrapped up Chapter 6. (Obviously I have ToB installed or I would have peaked at level 19.) The other party members are:
Keldorn - level 18, and the other main tank in the party. With boots of speed
and the Carsomyr, plus his true sight & dispel magic abilities, he is THE most invaluable NPC.
Minsc - level 18. Started out as a tank, but now I find he's filling on a more "support fire" role (he has a longbow Heartseeker +3 and Azuredge). However he can tank if needed.
Jaheira - level 14/14 fighter/druid. For druid spells, summonings and tanking if required (her AC is -11)
Imoen - level 7/14 thief/mage. Thief and scout. Plus mage of course.
Aerie - level 14/14 cleric/mage.
The kensai's shortcomings can be overcome by items such as amulets, cloaks and rings which can enhance AC. Having high stats at chracter creation help immensely - I started out with 18/77 in Str, 18 Dex, 18 Con, 18 Int (was thinking of d/c him to mage, prob. too late in the game now), 10 Wis, 8 Cha. Stocking up on relevant potions (eg potion of defence, of heroism, etc) also help to lower his AC.
Hope this helps.
Also, decide what weapons you want your kensai to specialise in and concentrate in putting points into that weapon. My kensai has 3 points in dual wield, 4 in katana, and 2 in long sword as these are the weapons and styles he normally uses.
He has chalked up about 50% of all the kills of the party. He is a mean killing machine and usually (for the average encounter anyway) is the solo tank while everyone else uses missile weapons or casts spells.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
