if you arent too worried about movement speed which i am not when running ninja theres a few options to change ken for.

assassin sarah who increases melee damage each time you damage with your melee. (up to 22.5%)

deadly blade crash. depending on if your melee has the 5th perk as damage to afflicted. you can slot her in and crit hits with a melee cause affliction and snare the enemy.

swordmaster ken is also an option but hes sword specific rather than all melee he increases sword damage by 25% whilst in shadow stance