What to do with a -def/+res Sigurd

Keep him. He has no need for resistance and will miss the defense very much. Nevertheless he's still a bulky magic tank and not a good skill fodder since he can only give away close defense.

I say keep him. The +res will help him take even less damage in combination with his b-skill and divine tyrfing. It's marginal, but not bad. -def hurts, but if you keep close defense on him he's still pretty tanky, and depending on the team you run, buffs will go a long way towards increasing his bulk.

If you do want to fodder him, close defense is a pretty good and rare a-skill, good for dragon slayers, units with mixed defenses, and units like lukas or dorcas provided you don't have any extra breaths to give away. Spd smoke is also pretty rare and useful on units with flashing blade effects (Ayra, Mia) or dagger units who already debuff def/res with any of their weapons.