Shamrock is the leader of the Holy Knights. He is the ranked Captain. His father was just promoted to Gorosei from Commander and we have no idea if he retains that title.
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I personally don´t think there´s any reason for Garling to give up his position as supreme commander of the Gods Knights because of his promotion, its not like he was probably out in the field doing missions anymore anyway, just giving orders, so we can agree to disagree on that, but he´s still a subordinate of Imu
And no, Doflamingo was a subordinate of Kaido, to the same effect of HKs being subordinates of Imu.
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I´m not sure how people came to this conclusion lol. Doffy runs his own crew and island, he was simply doing business with Kaido, not even an official ally with the man. Ceasar, Doffy, Kaido were all running their own islands, which is why Luffy fought all of them, they were the leaders of that arc. The Gods Knights in comparison, have to get slave tattoos where Imu can control their thoughts and behaviors, and can seemingly be killed by Imu on command.
What I was talking about was the reason he even exists. For years Luffy has made claims of wanting to fight Shanks, but we know Shanks is a good person entirely and very likely will be an ally in the end. Meaning for no other reason to utilize his design in an antagonist sense, Shamrock entirely represents a real motive for Luffy to fight "Shanks" in this way. I do not, for a second, think Oda is legitimately hyping Loki vs Shamrock outside of animosity generated like many other secondary characters have for villains. I.e. Wyper to Enel, Paulie to Lucci, Brook to Ryuuma, Jinbe to Hody, Law and Kyros to Doflamingo, Law/Killer/Zoro/Scabbards/Yamato to Kaido. See my point? They all end a different way.
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I´d say the difference here does exist, though. Shamrock was literally introduced trying to recruit and kill Loki, and the arc went on to go even deeper with their connections than that, through their fathers, history with CD´s and Giants. For comparison, it would be pretty hard to imagine Oda having a Luffy opponent like say Kaido, be introduced beating up Yamato or though.
Yasheen is kind of just reaching. It´s pretty obvious that Imu is the generic arrogant anime villain who isn´t going to try getting backup to help him out, since he thinks he´s a god or whatever, and Oda isn´t going to undermine his presence by bringing in new antagonists, as opposed to the three who have been on the arc since last year, fans just don´t like the HK on Elbaf, so they want Shamrock to return because they see him as more respectable, there isn´t really a serious argument in favor of it.