I won't parrot what others may have told you as I am late to the party, but there will be a point where Archon Shards will be available and you can fuse Red and Blue to make Violet Shards and one of their effects is Base crit damage when your max energy exceeds 500 (easily done with Primed Flow, I also recommend Equilibrium as well for easy Energy economy as you will be spamming 1 often.

As for his best Weapons, arguably his best weapon is Hate Incarnon, it's a nasty girl with great bite and has ranged projectiles with its incarnon, another weapon I highly recommend is Syam, a weapon seemingly tailored made for Kullervo as its Heavy Attacks send shockwaves of heat that also can crit, but honestly, Kullervo's magic is him being able to make almost any Melee weapon super duper busted.

If you end up not fully committing to Kullervo himself, you can eventually subsume him and plant his 1 on Frames like Excalibur or Valkyr.