A good Nohrian Trust build needs the support unit to have mostly proc skills. The best choices are Pavise, Aegis, Aether, Rend Heaven, and then one other(a good choice is miracle).
And then class depends on what type of stat boosts you want to give Kana. For example: A general will give a high Def boost, and a Swordmaster will give a lot of speed. So it depends on what stats you want boosted for Kana.
Base butler pairup stats are: +2 Mag, +3 Spd, and +3 Lck.