Monty Oum was a fan of BlazBlue. Mori is a fan of RWBY. The fact that Central Fiction had palette swaps based off of RWBY characters both shows that and it was a nice tribute, but Ruby clashing with Ragna the Bloodedge is beyond awesome. ArcSys really wants to pay their respects.
The fact that Ragna's official artwork has him smiling. Up until this game, none of Ragna's official artworks (even in spin-offs and guest appearances) depicted him smiling. If Ragna, who has possibly one of the most traumatic and scarring backstories in fiction, has something to smile about, things must be looking up for him.
The fact that this is the first game in the BlazBlue series where Ragna is finally given respect. Some jokes are still made at his expense, but never to the point of how it got in his home series; Yu and Ruby's interactions with him in their respective campaigns show them treating him as a Big Brother Mentor if anything. Considering all the shit most other BlazBlue characters have given him over the years, treating him like a pathetic joke at best and a blight that needs to be destroyed at worst, it's nice to see other characters give him some dignified attention.
This game is actually just one big Heartwarming moment for all the denizens of the BlazBlue universe, since characters like Nine and Jubei fight against each other to the death in BlazBlue Central Fiction, they can fight together as a team here.
The same goes for Izayoi/Tsubaki, who like Nine, was Brainwashed and Crazy since Chronophantasma, and ordered by the corrupt Imperator to kill her best friends in the game before. With Cross Tag Battle, however, we finally get to see how Nine and Tsubaki were supposed to act alongside their comrades, and it's great.
With Celica's reveal, this means that Nine can fight alongside her beloved sister, and Ragna, Jin, and Noel can protect their foster mother. Considering that this is also a timeline where Trinity resides within Platinum's body, Trinity Glassfile can talk to an old friend.
Ruby and Weiss's pre-battle quote, showing that Weiss has really warmed up to her.
Weiss: It's seems it's our turn, Ruby!
Ruby: Yeah! Let's do this!
Crossing over with funny, Ruby fangirling over Ragna's Aramasa is just plain adorable.
Ragna and Narukami's intro is surprisingly heartfelt when you consider Ragna is a bit rough around the edges. Narukami is polite to Ragna and encourages him to fight alongside him in battle, earning a rare moment of Ragna responding rather calmly that he'll fight with Narukami, causing them both to shout in unison: "Let's go!"
Their finisher counts too. Ragna tells Yu he doesn't care about his Power of Friendship but affirms that as long as Yu keeps his eyes forward, he'll watch his back. Yu then tells Ragna that they are already forming bonds. He even refers to him as "Ragna-san", showing how respectful Yu is to him and that he looks up to him.
When the Persona 4 VA cast were dubbing, they said it felt like a "high school reunion." Considering Persona 4 will be turning 10 years old not too long after the release of Cross Tag Battle, it feels fitting.
All of Hyde's "family" are present in the game with Vatista's inclusion.
In the cinematic intro, Ragna gives Jin a shoulder tap, before the two of them, alongside Noel presumably enter the fray. This is one of the warmest gestures Ragna has ever given Jin, and the fact that all three of them fighting together as a family is heartwarming as well.
Another from the intro. Even though Blake and Yang are both DLC characters, they are still present alongside Ruby and Weiss in the intro. Arc Systems clearly didn't forget about the second half of Team RWBY.
One of Ruby's palette swaps is a white color scheme◊ that clearly references Summer Rose, Ruby's latemother and Yang's step-mother. Depending on how one interprets it, this could be seen as Ruby donning the white hood in honor of her late mother.
Similarly, Yang's 13th color swap references her own mother. Even though Yang and Raven have a rocky relationship, it's still nice to team up two former members of Team STRQ in a way.
Going back to RWBY-based colors, everyone's 12th color is a RWBY reference. For the most part, they've all been pretty smart choices, but some even delve into Heartwarming and Ship Tease territory. Here are a few examples:
Makoto and Noel are Nora Valkyrie and Lie Ren respectively, and a look into Ren's childhood from Volume 4 shows him protecting a young, impoverished Nora from bullies, much like how Noel, alongside Tsubaki, stood up for Makoto in her past.
The four members of the Investigation Team included in the base game all represent Team RWBY, with their Personas representing fellow comrades. In User/Persona order; Yu and Izanagi become Ruby and Weiss, Yosuke and Jiraiya get Weiss and Ruby, Chie and Tomoe get Blake and Yang, and Yukiko and Konoha Sakuya get Yang and Blake.
The DLC takes it even further with Kanji as Jaune with Takemikazuchi as Pyrrha.
And Volume 3 takes it even further with Naoto's RWBY color being a reverse of Kanji's; that is: Naoto is Pyrrha and her Persona is Jaune. Arc System Works really wants to Ship Tease, don't they?
Aigis' RWBY-based color is Penny Polendina, naturally, with her being a Robot Girl and all, but what color does Pallas Athena get? Ruby, representing Penny and Ruby's strong friendship. The same also applies to Labrys.
Similarly to the Penny/Ruby example above, Aigis also gets a color palette based off of Vatista. Who is Pallas Athena, however? Hyde, the only person Vatista has a genuine friendship with. Labrys isn't given the Vatista/Hyde color combo like her younger sister, but she does get Linne/Waldstein instead, which is just as sweet.
Who all from UNIB receives Team RWBY's colors? Hyde and his friends. So far we've seen Hyde, Orie, and Linne as Ruby, Weiss, and Blake respectively. A little less heartwarming is Waldstein as Adam Taurus, though...
This is taken further with Mika as Yang. Sure, she's not close friends with Hyde like Orie and Linne are, but it means that Orie and Mika represent "Freezerburn".
And finally, there's Team RWBY themselves, who show their respect to fellow friends Team JNPR.
Amusingly, Hyde and his "family" get colors referencing Steven Universe of all things, but the prospect is still massively heartwarming, considering that much like Steven, Hyde is an outsider taken in and cared for by complete strangers who are later considered family by him. For reference, Hyde is Steven, Linne is Garnet, Waldstein is Amethyst, and Vatista is Pearl.
Chie and Yukiko get along well even now; in addition to having team dialogue, part of the reason Chie was busy looking for Yukiko in Episode P4A is because she wanted to share a beef skewer with her.
You wouldn't be dealing with Makoto Nanaya if you didn't get at least one of these.
It's really cute how much Makoto and Chie get along. In Episode P4A, the two of them bond almost instantly, and probably would have become besties if it weren't for the System.
Makoto and Yosuke also get along great and get featured together in the opening, with Makoto being one of the only characters other than Yu to never insult him in the game.
In Episode UNIB, Makoto, being the nice squirrel girl that she is, tries to befriend Blake. Sure, it's funny that they ignored Hyde in the process, but it's very sweet. Especially given how similar, yet different, the two girls are.
Chie and Waldstein's special win quotes really sound like they're warming up to each other, and Waldstein sounds like he has genuine fun fighting a normal sparring match for once.
Nu-13's RWBY-inspired color palette is of fellow Robot GirlPenny Polendina.◊ While at first this just seems like an obvious joke about their similarities, once you remember that Monty Oum was a huge BlazBlue fan and based Penny's fighting style (and possibly her origin as a Robot Girl) on Nu and her Murakumo Unit "sisters," it becomes much more meaningful since it means Monty's creation has come full circle; being represented by the character who inspired her in the first place.
Towards the end of Episode RWBY, Ragna saves Ruby from System's explosive entrance, injuring himself in the process. When the rest of her team arrives, Ruby asks Yang and Blake to protect Ragna and Es while she and Weiss fight System. Yang, on finding out Ragna protected her little sister, emphatically thanks him for doing so.
In Episode P4A, Kanji helps Ragna out simply because Ragna helped the Investigation Team out making him allies with them, and Kanji refuses to not help an ally out in a jam. Doubles as an Awesome Moment, as in this situation, Kanji's volunteering himself to participate in a fight with 3 of the BlazBlue universe's most powerful individuals in Ragna, Rachel and Hakumen without any fear, and after expressing a bit of worry over it, Ragna gives him the props he deserves.
Orie's interactions with Weiss are sweet, with both of them swearing to be good partners with each other. Special mention goes to her interaction with Jin, where, surprisingly, Jin ISN'T a Jerkass and actually shows Orie some respect. It could even be inferred that Jin's uncharacteristic niceness towards Orie is because he sees Tsubaki Yayoi in her.
Near the end of the Episode BlazBlue, Ragna has to fight Es and and a shadow version of himself. Since he needs a partner to participate, the one who comes to his aid is Jin, of all people. Ragna thinks Jin is here to fight him as usual, but surprisingly, Jin is completely cooperative and doesn't display any of his normal murderous intent towards his brother. Even after the fight, he boasts about how strong the bond between him and Ragna is and the latter, while embarrassed, doesn't deny it either. Goes to show that for all of their differences, they can cooperate with each other when push comes to shove.
Weiss Schnee and Yukiko Amagi may not interact with each other much in Episode Mode, but share a very warm and friendly disposition as a team. Even more adorable is the fact that Weiss helps Yukiko shill her Amagi Inn to their opponents after a victorious battle.
Weiss: "How did you like our dance? Did you enjoy the storm of fire and ice?"
Yukiko: "If the dance was to your liking, come to the Amagi Inn's hot springs to refresh your body and soul. We eagerly await your visit."
Hyde and Vatista, as usual, are full of this with their interactions. It's one of the few interactions where Hyde gets virtually no sass or awkward moments with.
Vatista promises that she and Hyde will spare their opponents.
Hyde: Guess I've gotta hit you with everything I've got. Vatista: No worries. I assure you will survive.
She also reassures Hyde afterwards. For context, overconsumption of EXS can cause an In-Birth to turn into a Void, meaning her line is more or less expressing concern for Hyde's safety.
Hyde: Sorry, I've never been good at holding back. Vatista: You still have your body, so you should be alright.
It's subtle, but their dialogue shows that they're quite comfortable fighting with each other, which is especially strange for Vatista, who is normally programmed to pacify In-Births.
Hyde: Man, I just wanna go home already... How much longer are the two of us gonna have to fight? Vatista: To borrow a human phrase, "God only knows." Though, "a bad idea is worse than none at all" may also apply.
With Yang's inclusion into the game, two players can have all four of Team RWBY fighting together in a battle which gives a special four-way battle intro, as seen here. It's a friendly sparring match to them.
In a similar vein, Nine's inclusion allows four of the six heroes to reunite (Jubei, Nine, Hakumen and Platinum). They also have the dubious honor of being the second group with a four way intro after Team RWBY.
Crosses over with Funny, but the Persona 4 Arena ending can also be this. The Investigation Team may have failed to stop System from creating a new world, but it is heartwarming to see the other main characters live normal school lives given theircircumstancesin theirhome universes.
Nine and Aigis' result line is both heartwarming due to Aigis' innocence, and hilarious when you consider who Nine's daughter is.
Nine: "A Persona, heavy artillery, a flight unit, and humanlike intelligence...My daughter would love you."
Aigis: "Is your daughter still a young girl? I am a protector of children's dreams!"
The EVO 2018 DLC reveal trailer shows off a special interaction between Akihiko and Mika where they fist bump each other◊. If that doesn't make you go "D'aaw" then nothing will.
Mika: My burning heart sets my chest ablaze!
Akihiko: My muscles roar as they break their limits!
Mika and Akihiko: Our fists shall collide!
Akihiko: And there's the gong!
Merkava's interactions fall under this, surprisingly. There are a few characters who understand his condition and react appropriately. For example: Jubei acknowledges his human side, pretty much reassuring him that he hasn't thrown his human nature away, and Blake even promises Merkava that she won't let him be harmed should he promise as well to not hurt others. Even the Detective Prince, Naoto Shirogane, recognizes his internal conflict and seeks to aid him. Just the fact she refers to him as "Merkava-san" sells it.
Because this game take place after Labrys's character arc, she is nothing but cheerful the entire time and takes up Yu's ideals of bonds and friendship. She is especially happy to fight alongside her sister Aegis and assures Yu that she's helping him out cause they're friends.
Tsubaki's greatest wish always was to fight by Jin's side, but was manipulated by the villains to oppose him and her best friends to further their plans. Now she can finally fight with him and her friends in this game. Doubly so if she's teamed Hakumen, who is also her idol and inspiration.
And as expected; Jin's behavior towards Tsubaki in their special interactions are nothing but sweet and heartwarming.
Similarly, Nine and Jubei's last interaction ended pretty tragically in their home series. Here, they can put that behind them and fight as both comrades and lovers, and Nine is still just as affectionate as ever with him.
Despite being a dick to nearly everyone he meets, Jin does have his moments. In-game, three of his friendliest interactions come in teams with Izayoi (given how intimately close he and Tsubaki are, this is no surprise), Mai (a holdover from his Academy days since he respects her and Makoto as his ex-classmates) and Orie (whose personality isn't actually that far off from Tsubaki's own). He also has some form of begruding respect for Mitsuru as well.