Sorry about delayyyy >___< But here is some feedback!
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General:
The character dynamic is interesting there: two childhood friends, one becomes a criminal and the other becomes a part of the local type of police force. That offers good dynamics! I vote for talking a lot about how much the situation troubles them. B|
They both are strong ninjas, I see. While merely being a strong character doesn't make a character a Mary Sue, I recommend you to make sure that you make them strong for the sake of plot and not for the sake of coolness. By that I mean that if the character is strong enough to make sense in the plot. For example:
- Usually you can't have S rank missions if you are a genin
- Usually the antagonist of the story needs to be strong in order to appear threatening enough for the protagonists
Otherwise, consider carefully how strong protagonists you need. Actually, I prefer protagonists who actually need to STRUGGLE and come up with clever plans to win the foe! If anyone can see that your character can win the foe by ease, expect to hear a loud "MEH!" from the audience.
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Medusa:
Her appearance makes sense, though her image doesn't reflect how her skintone is described in the description; the text describes it as ghostly pale, yet in image it is tanned. But that's just nitpicking. Age compared to rank make sense, too. Though... when did she BECOME a jounin? I think she needed to be in her home village to officially earn the rank.
An "elite member" in Akatsuki? I don't really like replacing any of the usual 10 core members of Akatsuki unless there is a good reason for that, but it's confirmed that they have other affiliates (for example Kakuzu's banking staff), so Medusa's position could be something like that. "Backup plan B?" Or wherever Tobi was claimed to come from.
But there is one big controversial thing: Rinnegan. It is considered as an UNIQUE doujutsu in the series, I don't recall exact uniqueness but at least Jiraiya was shocked to realize that Nagato had it. I don't dare to take a look at Narutopedia because of spoilers, but I recommend you to see what was stated in the series. If the series says something like "Rinnegan appears barely once in a thousand years", I see that it requires EXTREMELY careful handling to give it to Medusa now when Nagato/Pain used to have it only a few years earlier. It might be easier to come up with your own bloodline jutsu instead. Using Rinnegan for Medusa may poke people's eyes HARD.
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Shiguki:
Otherwise good, but an ANBU in age of 9...? Eh... I can't recall when Itachi did it (and I don't dare to check Narutopedia because of spoilers), but he wasn't younger at least and he was regarded as an ULTIMATE GENIUS. See what I wrote in the general section.