Is it really a Mary sue form when it's barely done much battle wise and ultimately lost to Alan's undefeated Charizard (which somehow... Isn't a sue, not that it IS im just saying there's just of a strong case one could make like everyone seems to do with Greninja :/)
Alan’s Charizard has lost several times, it lost to E4 Siebold’s Blastoise, got nearly killed by Primal Legendaries, lost to Lysandre’s Pyroar as a Charmeleon, was ineffective against Steven Stone’s Metagross despite having a type advantage. If Manon’s end episode segments are any indication, its lost to the likes of Mega Slowbro and even got put to sleep by Mega Audino too.
Charizard’s strength is warranted because Alan was and is always training 100% of the time, he’s always hunting keystone-holder, always collecting energy, always training with Alan, always always always since day one Alan said he wanted to be strong with Charizard and he only trained more when he resolved to become stronger to protect the one’s he loves.
Mega Charizard X is a Mega Evolution, anyone can have one if they have a Charizard, a Keystone and a Mega Stone and a powerful bond with said Charizard, even Trevor has a Charizard Y, Shouta a Mega Sceptile, etc. Satoshi is running around with a one-of-a-kind Pokemon form only he can use or trigger.
He’s Alan’s ace and ultimate partner, he’s Alan’s strongest Pokemon, Alan has gone through everything with him, Alan has been Mega Evolving him since early XY compared to Satoshi who only just recently met the frog. Charizard X is a mega evoluton and mega evolution is the face of Kalos. He and Alan actually have a strong, healthy and well displayed bond.
No case can really bad made of Mega Charizard X’s strength and success thus far because its all been fair and in canon.
Greninja’s fake form had a poorly explained lore, it’s inconsistent as crap.
Apparently this only happens and hasn’t happened in 1,000 years but SATOSHI somehow made it happen, even a village of people who specialize in ninja Pokemon and hail Greninja as some deity couldn’t even trigger this.
Greninja’s form was DEM in the sense it came at the most convenient times, you know how Alan would feel out his opponent then mega evolve moments later. Greninja almost always synchro evolved when he was losing, example, Shouta has it on the ropes then him and Satoshi press the win button in ten seconds with a one-hit KO. It drove Diantha, the strongest person and Champion of Kalos into a corner all on its own.
This form takes on his appearance, is named after Satoshi, Olympia apparently called it a new power, a whole episode of exposition was given to it as cheap and quick patch-up work because they realized their fuck-ups and they weren’t even consistent about that later on.
This form allowed Satoshi to one-shot his first fight in the league, beat Wulfric, beat Shouta, man handle Charizard X (while imperfected), and it was nearly impossible to revenge kill.
The explanation was that Satoshi cares so much for Greninja and vice versa that it transforms but this is BS when you realize, why does this only happen for Satoshi, does everyone in Kalos not love their Pokemon? Even then they did a terrible job convincing you that there was any kind of bond as for most of XY Greninja was just a battle ‘mon with barely any personality or characterization. Go watch the Korrina arc. with Mega Lucario and watch the Woods bullshit and tell me who feels like a character with feelings and emotions between Lucario and Greninja.
XY was just a letter for Satoshi worship, that’s why we got punching bag rivals, speedy fully evolved Pokemon for Satoshi only, nothing but Satoshi battle time and victories, constant Satoshi focus, Serena having a crush on him, Clemont looking up to him and taking a sound beating from him, Shouta thinking the world of him, Milefeui having a rivalry with Serena over him, Shauna-Tierno-Trevor all jobbing to and being astounded by him, group interactions centralized around him, Alan saying he’s “obsessed” with him, Pokemon-induced evolutions by virtue of Satoshi being their trainer and no hard work, many forced episodes only Satoshi saving the day and being glorified as a god damn action hero, and what is possibly the most overpowered Pokemon I have seen since Cynthia’s Garchomp in Greninja that only Satoshi can use.