"The platoons are in the formation you requested, General. Should I send them to support Captain Rex?" "I'll give the order when I give it, Sergeant Appo."
Appo was loyal to his commanding officer, even when placed under the command of Pong Krell, a Jedi Master who was known to be both ruthless and reckless. Nonetheless, Appo showed that his loyalty to his brothers and the Republic still came first as he did not hesitate to arrest and open fire upon Krell when he learned that the latter was a traitor.
Appo wore Phase II clone trooper armor that was painted with the standard blue markings of the 501st Legion, with the only sign of customization being the white arrow pointing down the midsection of his helmet.
—Appo, ordering his men to attack the Umbaran airbase[src]
During the Republic's assault on Umbara, Sergeant Appo was placed under the command of Pong Krell. Alongside the general, he witnessed Rex and his troops helplessly attempt to destroy a heavily shielded Umbaran Mobile Heavy Cannon. Appo suggested that his platoon should move out and reinforce Rex's but Krell rejected the request. After Rex's platoon was able to take out the enemy forces, Krell ordered Appo's platoon to move out and take the Umbaran airbase.
After Jesse and Fives returned from their attack on the Separatist Supply Ship, Appo, flanked by two Clone troopers, appeared and informed them that Krell wished to see them immediately.
"Captain, we've repaired the transmitter. It looks like it was sabotaged. We received a message from General Kenobi. His forces have captured the capital but the remaining Umbarans are heading here." "Get eveyone on the perimeter! We need to prepare for a full scale attack!" "Yes, sir!"
Appo was present during the execution of ARC trooper Fives and Clone trooper Jesse. Appo, similar to Clone trooper Dogma, did not object to Krell's decision to execute the troopers. However, unlike Dogma, Appo obeyed Rex's orders to remove Fives' and Jesse's handcuffs when the execution squad missed intentionally, unwilling to carry out the execution.
Appo later accompanied Rex, the 501st, and the 212th Attack Battalion in arresting Krell, who was discovered to be a traitor. Appo stood alongside the newly freed Jesse when their group barged into the control tower to arrest the Jedi Master. However, despite being outnumbered and outgunned, Krell was able to kill all of the arrest group, except for Appo, Jesse, Rex and Fives. Rex lead several men and Appo in capturing Krell, who had escaped into the jungles. Later, after the Jedi Master's eventual capture, Appo informed Rex that Krell destroyed the communications system and that all remaining Umbaran forces were heading to retake the airbase. Rex ordered Appo to get all troops to reinforce the perimeter, to which the latter obeyed without question.
Appo's unique white arrow customization on his helmet is a reference to Appa, a character from Avatar: The Last Airbender, a show that Dave Filoni worked on.
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