I know beliefs regarding mental illness are sensitive. I will try to interpret the metaphor and symbolism objectively, without inputting any of my beliefs, or what I believe Oda believed in there. We just follow the evidence… Anyway, let’s go.
Brook, a Hikikomori (shut-in) suffering from depression who needed help
A Hikikomori, or a Japanese shut-in, is a person with serious mental illness that you can’t just fixed by telling them to “man up and go outside already, geez”.
During the time of this arc being out (2009-2010), Japanese news media talked about these people a lot, and they are stereotyped a lot in anime, comparing them to otaku (also nothing wrong with being one). They are demonized by the media because the Japanese public thinks they are to blame for huge population decline in the last decade, which effects the economy.
Brook is not a Hikikomori by definition. Personally he wants to leave. He is a Hikikomori symbolically because he literally cannot go into the sunlight or he will be vaporized.
The circumstance is not up to him. He didn’t stay hidden in the shadow by choice. He does have a goal, as was revealed when Franky rudely asked him:
To be free, Brook has to win back a part of him that he had lost. He had to fight with his own shadow. Ryuma is symbolically Brook’s depression because it prevents him from going into the light. When he reminded Zombie Ryuma about the importance of his afro, he was reminding a part of himself why he must venture out!
Zombie Ryuma, Brook’s depression personified, cannot kill Brook. But he can destroy Brook’s identity, his afro.
In the end, it was Zoro that came and took on the fight in his place, showing that a friend can help you fight your depression. This is not the only time One Piece showed this. When Luffy was broken after Ace died, it was Jinbei who reminded him of his crew.
I also want to point out that Luffy invited Brook to join his crew immediately. This was his response.
This is very much like when someone asks to help out a depressed person. They don’t technically refused help, but they thought that they had to do it alone.
Moria and his fear of losing anymore nakama
Moria’s story is, if you think about it, is actually exactly like Brook’s. They both lost all of their crew members horribly and suffered severe trauma. Moria’s seemed to handle trauma of losing his crew a lot harder though, because he seems to no longer want human companions. He just wanted their shadows. Okay, so let’s define what the shadow means.
What are the shadows?
The only thing that missing when Moria took a person’s shadow is they cannot get into the light, which is a metaphor for depression, like Brook’s case. But the others aren’t depressed. They were downright pissed off. What else did the character lost when Moria took their shadow away.
Moria’s extracted shadow contain a mix of memory, instinct, personality, and skills. The people who got their shadow taken didn’t lose any memory (well, they went into a coma for 2 days), didn’t change their personality, didn’t feel weaker, or fight a bit worse than before… so what are the shadows?
The Shadow is Nothing.
Like, literally, nothing. The people who lost their shadow didn’t lose anything, just ability to go out into the sun. Moria thought it was EVERYTHING. He didn’t need to have a crew anymore except the three henchmen he really trusted that wasn’t a zombie. He thought that his zombie army filled with worthless shadows that contain fragment of what made a person human will do all the work for him. But it won’t. I’m not just saying that as if I am guessing an alternate reality where Moria went to re-fight Kaido with Oars+Luffy’s shadow. There is an evidence right in the canon storyline.
Sorry Nightmare Luffy, you look very cool, and the manga pages where you pummeled Oars were top notch. But symbolically, you are a cautionary tale.
There was a debate on Quora over which is stronger: Gear Fourth Boundman or Nightmare Luffy. It is true that Nightmare Luffy could stop Oar’ punch, and his enormous strength could throw Oars. We don’t know if the current Luffy can throw Oars. But as far as the plot of Thriller Bark, Nightmare Luffy represented a mistake. When I was reading it weekly, I wondered why Oda had the Strawhat redo the fight with Oars again. I thought Oda was afraid the readers will think Nightmare Luffy is cheating. Nope. It foreshadows how they will win the fight against Moria himself later.
Think. Why did Oda went out of the way to show that Nightmare Luffy knows how to use a sword? This is symbolic. If Nightmare Luffy can use a sword, why would he need Zoro? But Luffy needs Zoro. Live, human, real authentic Zoro. Also real Nami, real Chopper, everyone. The moment Nightmare Luffy couldn’t defeat Oars, did you noticed, suddenly all the straw hat become very confident. It’s like they know something the readers don’t.
Nightmare Luffy could have just continue pummeling Oars for 1 more hour and it would have still not gone down because a zombie doesn’t feel pain.
And this is why when Moria, with his delusional belief, absorbed 1000 shadows, and the chorus of this arc freaked out, Zoro confidently turn to them and said something along the line of: “dude, chillax, we won like 2 chapters ago.”
*Theory by Panasit