I want to get married!

[Gatts]

For the main question it is much harder to answer as it purely depends on the strategy you would like to use in KB. (As you divorce once, you can't get this poor girl back)

Xeona is nice as demons are very strong units + she has 2 weapon slots (very handy)

Undead zombie girl / Bandit princess - she can be changed, has very balanced slots, and if you want go for undead army (as I do so often) she is the must (+ 1 speed + 1 initiative for undead)

Frog princess / frog : +3 INT (not such a big deal) or +100% attack to snakes (would it be + 1 init/speed to snakes - would be better)
Also I would not go for the +xxx% attack bonuses so much - is underpowered (once I maxed by cheating my INT to 4000 -> used berserk on shooters, - should be + 2000% to attack - my priests had 480 or so attack, did 380 dmg instead 100 -> is this really worth?)

Neoka is beutifull and has slots that can hold very useful items (regalia) but well, not to mention that many tactics praise the elven units, like dryads (I don't like elves - are wusses)

Mirabella - may be beutifull, but I really can't imagine she could be usefull later on in the game (still some players have good tactics with pirates... again these would be useless later in the game )

I liked dwarven units - canoneer, giant... so then the dwarven girl would be my choise - with dwarven units you can fight till the game end (have huge HP/ resistance)... would ressurection work on giants - I would never use undead units... sad is that dwarven units desperately need some other units to be mixed up with ... than you have a lot of planning in your way to re-think which units go as prior and which as secondary.