First of all, the bad part: I took as completely lack of respect that BONES made us wait for about 3 months for the ending. Yeah, whatever, time slots, TV broadcast schedules, I don't care: We waited so long just because they realized after the first DVD/Blu-ray were selling like hot cakes. I don't care what you say, that's a fact for me. So they decided to 'expand' the ending and the episode probably was rewritten completely to the point where the amount of content could no longer fit into the time-slot assigned for the show.
I think given the nature of the show most of that can be ignored. If you liked this show as I do, you probably don't care too much about the missing content or the lack of 'what the heck happened here'. Why? Because the basis of the show was always the same, a very hilarious (and lovely) formula of a ridiculous amount of characters all involved in the 'daily' mess that Hellsalem Lot is. Leo is probably one of the most charming characters I have encountered in a while, because even though he falls into the 'Useless' Main Character category, he has a personality of his own and shows to be worth every minute of the TV show.
Side note: Yes, we could rename this show 'Leo got kidnapped! The animation'.
Anyhow, now, about episode 12. What a lovely mess! Of course there are some things that no one can stand against, first of all: the pacing, the missing content, and the lack of explanations.
What the heck are Leo's eyes? We can only do some conjectures, but none of them will really help to explain whatever happened here lol. Being these eyes some kind of God powers he can see stuff no one sees. It seems he uses his power to discriminate between Black and the King. I'm not sure if we're supposed to believe both are coexisting or that actually what the eyes do is to bring back the Black into the 'real world'.
What were the King's plans? Oh well, that's the difficult part. If I understand correctly the main reason was to be get out of the city and be free to rule Earth. Whatever he took from Leo it helped him to increase his power. However, linked to this, we found out that (sorry if I missed the foreshadowing, but I really didn't see it coming) White is actually the barrier herself.
So, the whole situation was a lose-lose for Black, since he allowed King of Despair to enter his body so he couldn't take over his sister White, and then it turns to be that White is what is stopping him from breaking the barrier. Regarding this: WHAT KIND OF PARENTS ALLOW THIS!!!11
So, Black 'kills' White, just to discover that's not enough. He will need to do MOARZ.
And then, well, what you see. The whole Kekkai Sensen gang goes to stop them. being led by our beloved Krauss-sama.
During his fight with King of Despair I think there is some sort of reference that Krauss has been living for a lonwg time as well, but please correct me if wrong. He says something like "even if you die now, the next you is going to keep you alive". I think this is referring to Leo -foreshadowed by the flashback where Krauss is with a baby (Leonardo?)-, but it also points out that he has some sort of vampiric/kingly power (which makes sense if we take into account how overpowered he is).
Later Krauss explains to ZETSUBOU-OU that they are not giving up in Black, whatever he does. Battle goes flashy and BONES, until Leo arrives. There Leo trolls hard saying he still can see Black in him, and in the middle of the mess he uses White (Ghost?) to separate the bad guy from the good guy.
Since the Barrier exists thanks to White she needs to be sacrificed to recreate it. Therefore, the ending, which by the way broke my heart completely: Leo "looking brave even while crying" ;____;
Them feelings.
Anyhow, very convoluted, much plot, very deep. I must confess that most of the script was made to look "deeper" than it actually is, which made me a little bit mad. But I loved the show, I liked the ending, and I think it was worth the wait.
AND OF COURSE I NEED A SECOND SEASON.