Ahh...what an eventful but sad episode. Quite a lot was shown, and quite a lot happens.
We go back to Shu and Gai's childhood where they first met and when Mana was still sane and alive. Compared to how they acted to each other in the past 11 episodes, you would find it hard to believe that both of them were good friends, but we all know they were and in a way, still are. It seems that Mana had been infected with the disease and Gai had found out, and as Gai said to Mana in that confrontation when he was younger, the crystals made people go insane, and thats exactly what happened to Mana.
Mana went insane. She threatened Gai in a way and tied him up, kissed her own brother, tricked Gai into shooting himself (That gun was rigged wasn't it? Or did Gai turn the gun of himself?) and ended up killing herself with her own power and many others when trying to cry out for help to Gai and Shu.
And of course, it happened in front of both Gai and Shu. Gai left to become stronger, where as Shu was traumatised and forced himself to forget as soldiers took him away from the wreckages which is why he always had fuzzy memories every now and then throughout the anime of Mana.
So, in the end Gai's void was pulled and it turned out to be a gun that forced other voids out of people's hearts. I wonder if Gai managed to change his void at the last minute when believing in Shu and holding his hand. It was a perfectly reasonable void to come from Gai. He could see Voids, he knew all about them, and in the end it was what Shu needed.
But, lastly, I thought I would have never said this before until I saw this episode. We see Gai's death which was in my opinion emotional. Impaled by a monster version of Mana, I could only cringe when I heard the crunching sounds as he was impaled and all we could see was his blank face. Then, to keep Mana from having her slumber distubedm he requested Shu to destroy Mana through him with Inori's void. He redeemed himself this episode when coming back to help Shu and also when the flashbacks gave some backstory. Shu's reactions were prefectly reasonable and I actually wished Gai was still around now...but alas...as he said himself, he was going to die anyway
You shall be missed by me Gai. A good death, much better than when a lot of us thought you would die back on top of that building in Shu's arms after getting slashed.
Good episode, one of my personal favourite of the anime so far.