This is going to be one long-ass post (Note: I'm going to call GG, which is her name, "GG-Chan"), also spoilers for those who haven't read the Zannen Jokanbu Black General-san volumes.
TNs (Optional; Which version of these TNs do you like better and your thoughts on it?):
Official-like DB! Thumbnail 1
Official-like DB! Thumbnail 2
The Original TN 1
The Original TN 2
Connections:
Two rather quick humanoid ninja protagonists that are "monsters" (GG-Chan being a hybrid monster girl/Pokémon, the original Japanese name, is called "Pocket Monsters") who are apart of a group that treated each other with respect and are supportive to one and another (The RX Organization/Ash Ketchum's Group) that wields kunais as their weapons (But Greninja has more than a Kunai) and are known for their immense speed and their variety of ninja-like attacks.
Back in their past, they were formerly not used to be a ninja and instead normal beings (GG-Chan being formerly a normal teenage girl/Ash's Greninja being formerly a Froakie) that they would usually trained that they needed to achieved to reach a goal (GG-Chan greatly admires Ninjas and tries to trained to be like one/Froakie preferred training alone to get stronger instead of socializing with the other Froakies), but soon finds out that there was an another way in order to achieving this goal (GG-Chan learning about RX Organization's offer of monster augmentation/Froakie realizing that it could only get strong with a Trainer it truly trusted), they would decided to go through this route, thus becoming the group's first ever (GG-Chan becoming RX Group's First Type 1 Proper Humanoid Monster/Froakie becoming Ash's first Kalos region Pokémon), also joining the said group after making their decision (GG-Chan joining The RX's Organization/Frookie joining Ash's Team)
The group also features a intelligent inventor whose inventions that, despite it working well, has backfires on them multiple times while some of them don't (Scientist-San/Clemont), they are also involves helping the protagonists with their inventions in some way (Scientist-San being the responsible for GG-Chan's Humanoid Monster Transformation/Clemont developed a special wristband-mounted device that would allow him to measure Ash and Greninja's heartbeats in order to invoke Ash-Greninja transformation)
After joining the group, they would eventually begin to trained their speed again (GG-Chan training in order to pushed her speed even further to be useful to the group/Froakie being trained by Ash to learned Quick Attack)
They helped alongside the protagonists stopping various enemies/entities during the story, ranging from a destructive being that would attacked others (Headgeared General-Sama's Clones/Professor Sycamore's Garchamp when in it's rampage during EP2), a robotic replica that would commit crimes to framed their original counterpart (Mecha General-san/Dark Clembot), the most well-known man but was soon revealed to be a manipulative villain (Mr. Brainwashed/Lysandre) that would captured and mind-controlled other beings that they would use to achieve their goal, include the protagonist's companions (The Secretary and Lightning Man/Z2 and eventually Squishy), and a evil dangerous organization of various criminals (The "Organization"/Team Rocket) that is leaded by a feared and toughed leader (Carcha/Giovanni) who were formerly apart of something (Carcha being the former member of/Giovanni being the former Gym Leader of Viridian Gym)
They seemed to get enjoyment from battling against strong opponents and would even developed a rivalry with them (GG-Chan would developed a bit of a rival attitude towards Braveman after their encounter, trying to get him to fight her after her training. During her battle against Black Mask, she enjoys the equal match against him and even said "You can fight! I give you that!", and later before their rematch, GG-Chan was determined to settled the scores with Black Mask once again/Greninja, after recognizing Sawyer's Grovyle's strength, would developed a rivalry with it. Even has been shown to enjoy battling and the opportunity against strong opponents multiple times)
During their journey, they met and fought against a black-colored character who they deemed as rival (Alain's Mega Charizard X/Black Mask) whose main attacks are centered around an element (Mega Charizard X's attacks being based around fire, and also having Thunder Punch by charging its fists with electricity/Black Mask's attacks being based around electricity by firing it out of his hands during against GG-Chan)
Both would receive a form that happened during their battle (GG-Chan's Special Ninja Suit, Black Kunoichi Armor against Black Mask/Greninja's Ash-Greninja against Diantha's Mega Gardevoir)
Both would later shared and formed a strong friendship with one of the protagonists (Minion 2/Ash Ketchum) after they showed kindness towards them (Minion 2 cheering up GG-Chan's sadness after calling her "cute"/Froakie was being shown love by Ash Ketchum), with the between of them showing caring and kindness towards each other, to the point of saving them, alongside their friends, from harm (Greninja caring deeply for its friends and would go to great lengths to keep them safe from danger/GG-Chan warning Braveman Black General's whereabouts and her goal of capturing the culprit behind the Mecha General-san's incidents, and later sending a message to Braveman where Black General is, thus saving her from Mecha General-san), while the latter also do the same to them (Ash Ketchum being known for saving his Pokémons multiple times in the series/Minion 2 saving GG-Chan by tackling her from Headgeared General-Sama's Clone's attacks)
Contrasts:
A character from a most well-known popular animated series VS A character from a obscure(?) manga series
An amphibians VS A insect (Amphibians, such as frogs, are known for eating insects with their tongues that came across in their path. And seeing how GG-Chan is modelled after a Cockroach and Greninja being a frog... it's kinda obvious. And surprisingly, Greninja is weak to Bug-Type Pokémons and GG-Chan IS technically a bug, seeing how she's heavily based on a Cockroach)
Froakie would leave or disobeyed several Pokémon Trainers for not meeting their standards before selecting Ash Ketchum as his trainer after seeing his bravery, while it only take GG-Chan for a simple transformation to signed up RX Organization's offer of monster augmentation.
While GG-Chan usually always training alone to increase her speed, Ash's Greninja would usually trained alongside Ash Ketchum during their training.
GG-Chan's Special Ninja Suit, Black Kunoichi Armor is just a black tight suit designed to insulate against electric attacks. Whereas Greninja's Ash-Greninja form occurs when Ash and Greninja's battle pulse overlaps during against Diantha's Mega Gardevoir.
Unlike Greninja and Ash who formed a strong friendship between each other, GG-Chan and Minion 2 would formed a relationship with each other for the last week
Animation Potential:
This is a Ninja-theme MU, ofcourse its going to be good. Why wouldn't it be? Due to their immense speed, I could see the battle being fast-paced.
You could have a clash between Greninja and GG-Chan's kunais as one of the another could overpowered each other. Before one of them knocked the other back and resumed the battle. And even have a moment where Greninja throw their shurikens at GG-Chan, who attempted to throw her kunais at them but were instead knocked away by the shurikens, causing her to get hit by it before getting back on her feet and continue to battle.
At the climax of the battle. Ash would activated Greninja's Ash-Greninja form and GG-Chan? Just give her... Black Kunoichi Armor. I guess.
At the final state of the battle, if Greninja wins, you could have him putting all his power into Water Shuriken before throwing it at GG-Chan who is in a vulnerable state 'till getting hit by it, causing a explosion that knocked her back into a wall before crumbling onto the ground. If GG-Chan wins, simply have her attacking Ash-Greninja from every angle at FTE speed before ending it off by slamming Ash-Greninja onto the ground with full force, knocking him out and turning him back into his original form.
Interaction Potential:
Due to the fact that Greninja is... well, a ninja. GG-Chan would be VERY excitement the moment she saw Ash with Greninja since she admires ninjas a lot, which is the reason why she joined the RX Group in the first place before engaging in a par with Ash's Greninja.
It's not gonna helped that the fact Greninja can't really talked... buuuttt Ash is there, and also Bonnie, Clemont and Serena too (And Minion 2 if you wanted to add him). Sooo... Ash, during battle, could compliment and be amazed/shocked by GG-Chan's incredibly speed.
When Greninja shows the same speed as her, she could respond on how Greninja is just as fast as her before determined to not lose to them in terms of speed.
You could have when Greninja started using his Water Moves against GG-Chan, she would be rather surprised by it before getting use to it overtime seeing now she gets around Black Mask's electricity attacks by wearing her black suit.
At the climax of the battle, both would get serious and focuses on the fight itself, attempting to defeat each other to see who would win or lose. And you can have GG-Chan telling Ash-Greninja the exactly same words she told to Black Mask: "You can fight! I give you that! BUT THE BATTLE IS ONLY JUST THE BEGINNING!", with Ash responding "She is giving it her all/best... It's time to give ours too!" as they continue to battle.
That's all I can think of it.
Music Potential:
The Statists Arguments:
Ash's Greninja MASSIVELY outstats GG-Chan in terms of AP/Dura because of the Wocky Pokémon's Game scaling, which is massive Ash's Greninja W and sad GG-Chan L to me...
...Buuuuuuutttt if you're the type of guy who believes that Pokemon Anime and the Games are completely separated continuity and remove all of the cross-scaling between the games and anime and then take the Anime Feats as its own due to this, then this is a fair MU!
Both scale around Multi-City Block level to Town-City level feats. By comparing GG-Chan to Rose Mist, who survives a MASSIVE explosion that reaches the clouds that left a HUGE crater afterwards, while the explosion, from what I can guess, should likely ranged around City level, making it on par with Greninja's clashes with Clemont's Luxray's Thunder Fang feat, which Clemont's Luxray is able to matched a boosted thunder bolt from Ash's Pikachu that done this and this feats. (Dunno if Greninja scales higher than City level). With Greninja being stronger AND likely more skill with Ash on his side..? Dunno.
But in terms of speed however, GG-Chan should be able to greatly blitzed Greninja via Braveman being surprised and shocked by her own speed. Braveman is able to reacted and dodge a laser beam fired by Black General. Thus granting GG-Chan's FTL speed. Also dunno if Greninja scale to FTL or higher tbh. Haven't watch the entire Pokemon Anime series yet.
GG-Chan may have problem with Greninja's Double Team and Aerial Ace, but seeing how she has great knowledge on ninjas and fought and overpowered a similar opponent that matched her speed and strength (Aka Black Mask). She can get through it... I suppose. But get totally destroyed by Ash-Greninja's Powered Water Shuriken.
But uh, if you do believe in Island/Multi-Continental/Small Planet level/Universial level Ash's Pokémon (Which I'm a bit iffy of, expect slightly the first one). Then uh... R.I.P. GG-Chan.
Anyways, the whole Pokémon's Game scaling kinda makes this a easy win for Greninja if you still want to include it. You COULD make it fair by just reducing Greninja back into Frogadier/Froakie where he is City level/Small City level but uh... that will just kinda get away the whole ninja-themed MU... and you COULD tried scaling GG-Chan to the Hyper Ray Gun's AP which has enough power to destroy a mountain (Which would make it fair against Pokémon's Game Mountain scaling). But no-one nor anyone has been shown tanking it nor have been stating to have their strength compare to it sooo...
So either way, it's debatable or not (With Anime-scaling only, Greninja would just stomped thanks to Tyranitar's Mountain feat when you bring in the games when GG-Chan is only just City level)
Conclusion:
Overall. I personally think this is a good alt MU for Ash's Greninja and the One for GG-Chan, still prefer Ninja-Trained Siegfried for Ash-Greninja tho.
Any thoughts on this MU? And also sorry if I got something wrong or if the connections feel strenched btw.