|This build aims to utilize Jakob's impressive bulk and access to Fighter skills in order to turn Jakob into a very strong mixed phase unit capable of fighting on both Player and Enemy phases.
Jakob's optimal boon is +Atk and optimal bane is -Spd. This is because Jakob cares most about being able to hit hard, and then he uses his skills in order to always double, thus making his (already low) speed completely pointless, and thus safe to lower.
Slaying Bow+ is Jakob's weapon of choice, as it lowers the cooldown on his specials, which help enable Jakob to quickly dispatch his enemies. Overall the +Def refine is the most useful, as it provides more stats that +atk in the defensive stat that is more commonly hit.
Pivot and Swap help Jakob get into position better. However if one runs Armor March 3 Reposition becomes a strong tool.
Jakob's special choice largely depends on the direction you want to take the build. Ignis will activate on any player phase combat where the enemy counterattacks. This lets him cut through many high defense units such as a lot of the armors and most of the dragons. While his enemy phase suffers a lot more due to not having an instant special, it still is able to counter a lot of enemy phase focused sets.
On the other hand Glimmer offers a lot of instant burst on the Enemy Phase, which is very nice against magic users and other squishy heroes that try to initiate into Jakob. However, your Player Phase special proc will be Glimmer, which hurts Jakob's ability to take down higher defense units.
Close Counter enables Jakob's enemy phase, allowing him to counter attack vs melee units that attack him.
Bold Fighter 3 lets Jakob invalidate his low speed on Player Phase, giving him the ability to automatically follow up, in addition to lowering the cooldown of his special so he gets access to it faster. This is what enables Jakob to have such a scary player phase.
Jakob's C choice is also dependent on team composition. Without other armors Def Ploy 3 allows Jakob to utilize his high resistance to both enhance his combat as well as his ally's. However, if you run other armors Armor March 3 is an obvious choice in order to give Jakob and the other unit more movement options.
Jakob's Seal of choice is Quick Riposte 3, as it allows Jakob to activate his special on enemy phase (whether it's a second Glimmer or the first Ignis). Generally Jakob is durable enough to tank two hits.