Highlighted weapons are free options.
Yun Jin is a Support character for Normal Attack focused characters who can deal off-field damage via her Burst. Yun Jin's Burst grants nearby party members 30 stacks of additional flat damage bonus on Normal Attacks which scales with Yun Jin's current DEF.
This means that external DEF buffs like from Gorou's Skill and/or Burst doesn't provide much to Yun Jin since it only buffs active characters and not off-field ones.