It's a loving deconstruction of sentai superhero, mecha and other classical anime genres. It's quite clever. It's also by Satoshi Mizukami, who sounds like an extremely interesting creator and has done manga like Spirit Circle and Lucifer and the Biscuit Hammer. However this show's still basically about fight scenes between piloted giant robots.
Souya Kuroi is a short-tempered schoolboy who lives with a intelligent purple cat called Sensei and a hot green-haired maid called Ginko. Souya's a decent lad, but he has a few quirks. Firstly, the amnesia. Secondly, he wants revenge for the destruction of his home planet. (He lives in Tokyo, by the way.) Thirdly, every so often he'll get into a giant robot and have battles.
This show is a massive retro rush. Look at the title card fonts, for instance. It's a throwback to those 1970s children's shows in which Mazinger Z et al fought evil every week. That's fine and I approve of this kind of joyful, non-ironic celebration, but it's also a problem because giant robot fights are boring and yet that's what the show's built around. It's a pretty simple show, really. It's being deceptively clever underneath, with lots of slipperiness about "goodies vs. baddies", "revenge vs. forgiveness" and so on. I particularly admire the last episode. There's a humanism here that's subverting all kinds of genre expectations. However all that's underneath an apparently bog-standard giant robot fight show.
Every episode has a battle. Surreal giant alien menaces keep flying in from the ocean to "seal" the Earth. To be honest, I wasn't that grabbed by a lot of this, even though I could see and approve of the cleverness underneath, e.g. unexpected character motivations or the questioning of assumptions that normally go unchallenged. The show's adhering to a formulaic genre storytelling structure (i.e. children's anime about giant robot fights), which on the simplest level makes it a bit mechanical itself.
Going into the last episode, I wasn't sure whether to keep or delete the show after I'd finished. It ends well, though, and I eventually went for "keep".
That said, though, there's still a lot to admire here and this show was the Best of the Season (or even of the year) for a quite a few reviewers. Firstly, the show's playing games with the concept of heroes and villains. There are at least three sides here (Pacifist, Nebula, Grand Paladin) and none of them are evil. They're all noble, good-hearted people who love their friends and just want to protect everyone in the universe. Well, except for Souya (i.e. the show's protagonist) and even he's got a lot of learning ahead of him. Souya's fighting to protect Earth, but at the same time he's not. He wants revenge, but this concept's getting dissected almost down to its atoms and we'll see contrasting angles on it. Sirius vs. Riel. That's all I'm saying. Ginko's "ieta" in the last episode destroyed me. Similarly, "they destroyed my planet" is a bit of an SF adventure cliche and it's often used glibly (e.g. Alderaan in Star Wars), but look at ep.8 here.
The plot also moves fast. It does three season finales in twelve episodes, each one feeling like the absolute, definite end of the series. Late in the day, there's a five year time skip. Characters grow up, literally and figuratively.
There are clever little things that amused me, as well as the clever big ones. Ultimate superboost secret weapon attacks sometimes have a price, but Souya's is funnier than most. (You get a hangover. This made me laugh.) There's also a sentient alien dog who's being voiced by the Great Norio Wakamoto, aka. anime's most famous ham, even though half the time he's just saying "woof" (or rather "wan").
In the end, I kept it. I admire the show. All the characters matter, with no one at all being a one-dimensional throwaway. Ginko is fantastically important. Nozomi is wonderful. Grandad is triumphant. You could draw interesting comparisons with Evangelion, even though that show is clearly more mature, complex and bleak in its deconstruction of the same genre. However at the same time it's also a somewhat repetitive series of episode-by-episode fights between goofy robot monster suits. It's still very clever, though.
"I'm sorry. You can take revenge on me if you like."