Judging from the wanted poster size comparison, he/she looks like an average person, not large or not even a big one.
The government keeps her/his identity as a secret, could be related to;
- Safety / security reasons because she/he isn't strong and could be assassinated
- He/she is not from this world (an alien) so the people, Celestial Dragons, and the majority of the Marines wouldn't like it,
- He/she is an immortal due to Ope ope no mi youth surgery, and again due to security reasons they keep her/his identity as a secret
We also don't know if the upper echelons of the Marines and CP0 (though considering CP's position they probably know it too) knows about him/her or not, Doflamingo said that he learned the ''secret treasure'' in Marijoa, thus the Celestial Dragons were chasing him and wanted him to die, and they gave him a seat in Shichibukai so he could collaborate.
If the secret that Doflamingo knew was Imu, then that also means other Celestial Dragons knows about Imu as well due to chasing and wanting to kill Doflamingo because of learning their secret,
This is where the things are getting interesting, because we know that the Admirals are the ''direct subordinates'' of the Celestial Dragons as Sengoku said, which means that they might know the secrets that Celestial Dragons also knows, so all Admirals and Fleet Admirals might know about Imu if other Celestial Dragons and Doflamingo knows it.
CP0 is also highly possible, considering they are even more close subordinates to Celestial Dragons than Admirals are.
What about Garp? If the information about Imu is only exclusive to the ''direct subordinates'' of Celestial Dragons (who are CP0 and Admirals), and Garp also didn't want to be an Admiral due to not wanting to be a direct subordinate of Celestial Dragons, then it's lucky for World Government that Garp didn't become an Admiral, he might leave the marines after learning, or he also knows it like the other Admirals, but he simply doesn't care.
The information might be only exclusive to the highest ranked Celestial Dragons like Gorousei and their other highest ranked potential replacements, then Admirals and low ranked Celestial Dragons might not know about this as well.
Also about Gorousei, why they are only 5? Why not 4 or 6? What's the thing about 5?
Their structure might be similar to Revolutionary Army's structure;
As you can see from Revolutionary Army, which opposes directly the World Government;
- East Army
- South Army
- West Army
- North Army
- G Army (Grand Line first half)
If every member of Gorousei is responsible from one area (like South, East etc) then it make sense that they are only 5, just like the Revolutionary commanders.
And the one who is responsible from the new world is no one (since the Emperors of the Sea, Yonko controls that area), or represents Imu itself, if the learning the secret about Imu is on the last new world island; Laugh Tale.
As for the question why that Gorousei obeys Imu, if Imu isn't a top-tier? Is that mean that Imu is %100 a top-tier?
I think this is a good question, but still there could be an answer for that it doesn't necessarily mean Imu is %100 a top-tier (although it's still highly possible),
Imagine CP0, and Admirals. They can one-shot Celestial Dragons, who are fodders. But they still obey Celestial Dragons. It was the same with Spandam, who wasn't even a Celestial Dragon, and CP9 was obeying the orders from fodder Spandam.
If Gorousei are chosen from the high ranked Celestial Dragons, and if they were trained from the youth (just like CP9), then they might be loyal to Imu just like CP0 and other CP agents were loyal to fodder Spandam and Celestial Dragons. This could be an explanation on why they obey Imu if he/she is a fodder. And we know that Admirals also obeys Celestial Dragons as well (except for Garp). Although this might not be the case, Imu can still be a top-tier.
Some tags;
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