Fighter HP: increase
Fighter DEF: increase
Fighter ATK: increase
Seo-Yun is a must get for those using fighters as their primary troops. All of her troop skills benefit fighters, so she should be upgraded to 131.
Her level 71 Hero skill Swift Steps is amazing. It increases dodge by 20% making her one of the harder heroes to hit in duels and campaign modes, for that alone she is a top-tier hero and could easily be upgraded to max. Her single target damage is nothing to sneeze at either, hitting for 300% damage.