It turns out that the concept of an unbreakable sibling bond works when it's comedy because it's just too absurd to be realistic and it's fine then. take away the surreal factor and it just becomes over-the-top melodrama: Riku and Schwi are so very much in love in this film but why exactly? Why would a robot with no capacity of emotions fall in love with a human; why would a human answer these feelings when the robot has next to no personality? How exactly developed these feelings? The film explains it not and most of it was just over the top melodrama — boundless love apparently only works be it made absurd enough. It was genuinely cute and endearing in the main series whereof I'm a fan but this is basically like watching Titanic all over again.
Plot-wise it's mostly filled with holes and weird explanations as to how exactly Riku and Schwi could stop the killing. At the end Tet just appears because Riku wished for it and becomes the god because that's how it works I guess. Schwi just grows a massive armored battle-suit around himsefl at the last moment because he can I guess; the Ex-Machina just decide to help Riku because "they are tools following orders" with no real explanation. It's a plot filled with inexplicable nonsense plot turns that is strung together by extreme melodrama of one-true-love stuff.
It does however tie into the main series and explains a lot about the history of the world so that's nice but it's a complete break of style from the comedic and lighthearted tone of the main series; what one gets in the end is a broken nonsensical plot and extreme melodrama. It's probably up your ally if you like stuff like Titanic though (in which case your taste is vile).
As far as the animation and battle-sequences, visuals and all that stuff no one cares about goes: those are actually very good.
