Ever wonder how Gai can be such a charismatic leader and read people so easily even though he is the same age as Shu? His ability is to see people's Voids. People's Voids represent their personalities given form. Thus, he can infer their personalities from what he sees.
When did things start to go really wrong in the show? Around the real-world Christmas of 2011. Another Lost Christmas.
When Mana starts infecting the whole world through dance, why would the show make her do something that is completely unrelated to her character? There is no indication prior to that moment that leads the audience to believe she is a talented dancer or even liked to dance. However, Mana and Inori are polar opposites, split personalities inhabiting the same body, so to speak. Since Inori is able to stop the progression of the virus through her singing, it makes sense that Mana would be able to spread that very disease through dancing.
Hare's Void appears to be a manifestation of her "warm and caring personality", but there's more to it: remember - her first love was the Kind King from the fairytale, who ended up destroying his kingdom by being overly generous. What is her Void? An instrument of healing others which puts her at great risk and eventually leads to her death. She can only give, until she dies, and never sees reward.
Yahiro's Void's One-Hit Kill ability reflects his desire to end his brother's life, but why do nonlethal hits inflict the apocalypse virus? It's due to his brother's own infection. It's possible that a bitter desire of Yahiro's is to inflict onto the world the suffering he has seen his brother go through.
Why is Shu blind at the end of the series? Because his Void also allows him to draw the pain of others unto himself, and this is exactly what he did for Inori before she perishes.
Some people might be surprised that Rowan would be able to see any "good" in Daryl. After all, Daryl is a bastard with no qualms about killing and with a psychopathic streak. But when you think about it, this is a series where Good Powers, Bad People is not in play, and his Void is one which reflects that which others do unto them. And considering what we know of his father, it is very likely that his behaviour was a result of Parental Abandonment of a kid who just wanted his father to acknowledge him.
If your void represents your heart/personality, it would make complete sense that if/when it is outside your body and in the off-chance that it's destroyed, that "you" cease to exist, and thus die. Hare, from the moment the bandages broke, was already dead, for everything that made her "her" was destroyed. Much like how the Specters from His Dark Materials devour someone's soul, leaving them an empty shell. Essentially dead.
While it's obvious that she doesn't particularly mind being a paraplegic, even if she doesn't necessarily like it at times, it stands to reason that Ayase wouldn't mind being able to walk again, either. Why not use Hare's healing Void to fix her spine?
They're either paying respect to Ayase's belief that being paraplegic is part of what defines her, and thus taking it away would make her less like herself; or Hare's Void can't heal severe physical impairment that's been in place for past a certain period of time.
Because using Hare's Void for that means accepting help from others that is given ... pretty much out of pity. She doesn't accept help, and especially not out of pity. It was only near the end, when Hare was no longer alive, that she accepted help and even then it was Shu whose help she accepted, not Hare nor anyone else but just Shu.
So much drama is the result of Souta being an F-Rank... but his Void is actually exceedingly useful, not to mention hideously powerful. Sure, it says that it's just for "opening things" but, evidently, this is accomplished by simply annihilating them, and everything can be opened thus. It's basically a miniature uber-cannon that was always used by idiots who didn't understand it's potential.
Shu outright uses it as a weapon against Gai in the finale.
From what we know, the ranks of voids are determined by their power level. It's not really touched upon how that influences what they can do, so it might be that his void can't really do that when used by Souta and only when Shuu does it ... or it might be that it's simply a void with a great effect for a low power level and their way of ranking is just that bad.
Gai never 'does' explain what he's fighting for, exactly, but Shu joins him anyway. Apparently, it having something to do with revenge-killing GHQ personnel is enough for him.
He is hanging around Gai because A) The GHQ know who he is now, so he might do it for protection B) Gai knows where he lives, so Gai was subtly manipulating him to come back.
What happened to Shu's Void Genome power? At the end of the series he's just going around with a regular prosthetic arm, despite there being absolutely no indication that he lost his power in any way.
It was stated that the reason Void exist is because they area all part of the Apocalypse Virus, which was destroyed thanks to Inori's death, so his powers are gone. So Shu's Genome powers are nonexistent.