Back From The Dead: Yomi, as seen in the Ending credits.
Beware The Nice Ones
Big Bad: Ezra Xehanort.
Big Damn Heroes
Bittersweet Ending: Everyone Lives at the end but at the sacrificed of both Ayano (who stays in the Daze at it's destruction) and Yomi. And it's reveals that all the Mekakushi Dan members lost their Eye powers and the ending itself reveals Yomi, back in his original gender, is Not Quite Dead.
Break The Cutie
The Cameo: The main protagonists of Revenant Tales/Pokémon makes an appearance in The Stinger of the final episode.
Darker And Edgier
Deus Ex Machina: Possibly, given how the ending shows Yomi (in her original gender) Back From The Dead from her Heroic Sacrifice at the end with no explanation or whatsoever. Though it's possible Arceus is the one who brings her back to life at the end.
Even Evil Has Standards
Expy: The Tartarus' master and Big Bad, Ezra is one for Xehanort from the Kingdom Hearts franchise who also happens to be the Big Bad as well... The only difference being Ezra is a lot more sympathetic than Xehanort is.
Narita himself is basicaly a Gender Flip of Nui Harime from Kill La Kill right down to their personality and involvement into one character's relative's murder (Natsu's mother, Serge and Ryuuko's father, Isshin).
Face Heel Turn: The Mekakushi Dan (except Mary, Seto, Hibiya and Konoha) does this near the middle of the series. Although in an unusual case, it's not because they join the villains' side and more like Screw This, I'm Outta Here! reason. Both Keith and Reflet (of all people) calls them out on this and it took several arcs for them (especially for Momo) to realize what they did.
Gender Bender: Yomi reveals to be this, and she willingly does it too! Given that he changed his gender into a female in order not to reveal his true identity so he can protect Elize from harm.
Good All Along: As revealed in the final arc, Reflet is actually one so it's no surprising that most of the main character's are surprised at this, considering how his personality comes off. Roku even lampshades on how the hell Reflet supposed to be the good guy.
Heroic Sacrifice: At the end of Xehanort's Ascension arc, both Ayano and Yomi does this at the end. In the former's case, she willingly stays in the Daze during it's destruction after using her Favoring Eyes' power to Ezra and the rest of the Mekakushi Dan while the latter sacrifices himself to bring back the Dan's memories (excluding Shintaro) after they lose all their eye powers.
Luckily for Yomi, he's Back From The Dead as seen in the credits.
Impaled With Extreme Prejudice
Mama Bear/Papa Wolf
Moral Event Horizon
Nice Job Breaking It, Hero
Not Quite Dead: In Tartarus arc, it's revealed that Ruby is alive after performing a Heroic Sacrifice with Sapphire in Revenant Night arc of the last series after Roku and Shintaro stumbles upon a ruin building with him unconscious in there.
Also Yomi, as seen in the finale after her Heroic Sacrifice with Keith passes by a familiar figure resembling Yomi's male form only to be revealed after he turns around it really is Yomi.