I would generally leave most of his abilities the same. Maybe remove the health requirement from his basic attack so that it always has the 80% chance to stun. I think making his special better by either giving a retribution or tenacity buff would help out. Then I think he just needs a 4th unique that gives Emperor Palpatine synergy. My idea Royal Guard rework would be:
Force Pike: Deal Physical damage to target enemy with a 50% chance to inflict Speed Down for 2 turns. This attack has an 80% chance to also stun the target for 1 turn.
Imperial Phalanx (3 turn cooldown)
All allies gain Defense Up for 2 turns. Empire allies gain Health Up and Tenacity Up for 2 turns. Royal Guard gains 35% turn meter.
Unyielding Defender
Whenever any ally takes damage, Royal Guard gains 15% Turn Meter and Health. In addition, Royal Guard Taunts for 1 turn whenever another ally falls below 100% health.
Imperial Guard
While Imperial Guard is alive, Emperor Palpatine has bonus Health and Protection equal to 35% of Royal Guards max. Whenever Emperor Palpatine takes damage, grant Royal Guard Offense Up for 1 turn and Royal Guard has a 50% chance to counter attack any opponent that damages Emperor Palpatine.
Basically I just want Royal Guard to be buffed to make himself and the Emperor great again.