Since Yukong is the first 4-Star character dealing Imaginary Damage and could thus be your only character of that damage type, it might well be worth boosting her damage output. For the Planar Ornaments, you can either go with an option supporting the entire team or one that increases the Energy Regeneration of Yukong, enabling her to use her Ultimate more often.

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Best Relic Sets for Yukong:

Wasterlander of Banditry Desert (4) – +10% Imaginary Damage, +10% Critical Hit Rate when attacking debuffed enemies, and +20% Critical Hit Damage when attacking imprisoned enemies.

Best Planar Ornaments for Yukong:

Fleet of the Ageless (2) – +12% maximum HP, +8% Attack for all allies if the wearer’s Speed surpasses 120.

Sprightly Vonwacq (2) – +5% Energy Regeneration Rate, the wearer's action is Advanced Forward by 40% at the start of battle if their Speed is 120 or higher.

Best Planar Ornaments for Yukong:

Fleet of the Ageless (2) – +12% maximum HP, +8% Attack for all allies if the wearer’s Speed surpasses 120.

Sprightly Vonwacq (2) – +5% Energy Regeneration Rate, the wearer's action is Advanced Forward by 40% at the start of battle if their Speed is 120 or higher.

Prioritize the following Relic and Planar Ornament stats for Yukong:

Head – HP (Primary), Attack, Speed, HP, Effect Resistance

Hands – Attack (Primary), Attack, Speed, HP, Effect Resistance

Body – Attack (Primary), Speed, HP, Effect Resistance, Attack

Feet – Speed (Primary), Attack, HP, Effect Resistance, Attack

Planar Sphere – Imaginary Damage (Primary), Attack, Speed, HP, Effect Resistance

Link Rope – Energy Regeneration Rate (Primary), Attack, Speed, HP, Effect Resistance