After almost 700 ranked games of Kokoro here are my comprehensive thoughts on her in DOA6, TLDR at the bottom:
State of Kokoro:
I hate to say it, but I do think that Kokoro is definitely a lower tier character in DOA6. Not garbage, but far from being a strong pick. The primary reason for this is that her character identity is COMPLETELY at odds with the sidestep meta and the new gauge system. It isn't impossible to win games as Kokoro, but it's usually through abusing the enemy character's weakness or punishing sidesteps, not through the strengths of your own character.
So most of probably know that I fucking hate the sidestep attacks, but it's definitely worse for Kokoro compared to some other characters. Let me explain why I think this is the case.
Most of Kokoro's offense is through her heichu mixups and her launchers. None of her launchers track, and since stuns are weaker in DOA6 there are few times where I feel like I can actually get a guaranteed launch. It doesn't help that she only has a single fatal stun (no sitdown stun). So with launchers hard to guarantee and easy to sidestep out of, how are the heichu mixups? In DOA6, Kokoro got a new mid kick from neutral AND out of heichu. This mid kick and the fact that her fatal stun tracks are the only things preventing her from being a trash tier character. This kick is the only tracking move out of her heichu stance (unfortunately the stun isn't as good as her mid or high punch options), her other 3 options do not track and get beaten by sidestep attack. However, this means that instead of worrying about 5 options out of Heichu (Low, Mid Punch, Mid Kick, High Punch, Delayed Regular Throw) they only need to worry about 3 (Any punch, Mid Kick, Delayed Regular Throw). The existence of the sidestep attack means that Kokoro actually has FEWER options out of Heichu in DOA6 than she did in DOA5.
Another issue with Kokoro right now is the fact that the meta diminishes her strengths and punishes her weaknesses. Kokoro stylistically is a character that wants to stay in your face and constantly pressure you with mixups at close range. However, you can't really play this overwhelming offensive style in DOA6, and it is much easier for opponent's to deal with pressure compared to 5. Sidestep attacks, close hits, and Break Holds are all new tools that are great at creating distance and disengaging from the enemy. These tools make it extremely hard for Kokoro to stay close to the enemy like she wants to. Once Kokoro is out of your face and at range she is fucking awful. She has no low sweep, no long range single move gap closer, her 66P (main initiating tool from DOA5) loses to sidestep, and her throws are pretty short range. Kokoro has one viable gap closing move, 46PP. This move is the only one that is reasonably fast, covers a good distance, and the second part of the string tracks so it can beat sidesteps. Her other gap closers are not useful. (Uncancellable charged 1PK which can be sidestepped anyway and a very slow kick string that puts her in back turned for half of it, also doesn't track until the end). I have recently discovered that her 7P->2K->P string covers a surprising amount of distance, but it doesn't track either so it might not really help. With these garbage tools from range, Kokoro is at the mercy of anyone good at range or with offensive holds.
I can already hear people yelling at me that "sidesteps are punishable" and that "you just need to play around it" but hear me out. The problem with Kokoro is that you need to drastically change your playstyle to deal with the weaker stun game and the sidestep spam, whereas other characters already have ways to deal with these issues within a single core playstyle. That's the main issue for me, some characters can play the exact same way whether their opponent likes to sidestep or doesn't sidestep (low sweeps, offensive holds, tracking strings that lead to guaranteed launchers), whereas Kokoro has to switch from pressuring down the opponent with long strings to playing super passive and using a different set of tracking moves. It isn't impossible to win on Kokoro, the problem is that to win you have to seriously outplay your opponent or rely on them messing up. Most of my wins are off of throws now because regular attack strings are so unreliable.
What I'd Change:
Give her a low sweep for the love of god, or any tracking low move. A sit-down fatal stun would also be nice, just put it on her 2PK so it isn't a useless move from neutral anymore. It would also be super nice if one of her Heichu punches could track, since all three can be sidestepped right now. These changes could probably be enough to push her into average or even slightly above average since her throws are so strong. The stuff I list below are more biased things I'd like as a Kokoro main.
Make her 1PK cancellable, since it's only good for cheesing people on wake up or as a shittier version of Leifang's backstep punch thingy. Give her a high punch option at the end of her 1PP string, it's already super slow and telegraphed so it wouldn't hurt to have another option. Finally, it would be nice for Kokoro to have a three kick string from neutral since a bunch of the female characters have one. It could either be an extension of a 3K string or maybe something out of 4K from neutral.
How to Win with Kokoro:
-Heichu is still decent for opening people up, the punish just isn't as reliable anymore. The kick is usually my go to option since people aren't used to it (they won't hold) and it also tracks (beats sidesteps).
-Launchers are a bitch because low hold spammers make you whiff 4PK and 7K leaves your back turned. Ideally don't use the 7K launcher, since the back-turned makes sticking the heichu air throw pretty hit or miss, it's more reliable out of the other launchers. If you do get a launcher, try your best to get the air-heichu into PK, it's a close hit and is almost always the best option in the air.
-P-2K-P/T is your bread and butter from neutral. The low kick is super fast and pretty much no one blocks or holds it, it's some nice pseudo-guaranteed damage and usually when they get hit by that the heichu throw lands. 9PP is OK but it whiffs against low hold spammers.
-Whenever possible try to land your fatal stun, it is the only move that guarantees a launcher (unless they breakhold after) and it also tracks.
-Most importantly, use your throws ALL THE TIME, one of Kokoro's saving graces is that her throws do a TON of damage and with the prevalence of sidestep spammers and OH users running around most of your damage will come from throw punishes.
-Don't be afraid to sidestep as well. It's a dick move but you are handicapping yourself if you don't use it.
-Using break blow from neutral is actually not awful, just a bit janky sometimes. Since her range is so shitty it actually functions as one of her gap closers.
-Her new chain throw is really good, and most people won't know to counter at the end so you can usually guarantee a bend over stun at the end to set up another combo.
Everything Else:
-Ponytail and Pigtails are tied for best hairstyle, followed by sidetail, beret long hair, and regular long hair.
-Best costumes are in order: Casual, Deluxe, Default, Schoolgirl, Exercise, and Kimono.
-Her new face is honestly fine she's still cute
-She got shortchanged in the story but she got more development in 3 chapters than most of the rest of the cast so 6/10.
-Her encyclopedia and in-game dialogue make her seem very cute and delicate but in the story she's got serious anger issues
-Her outro pose is god tier, her character select and intro are meh
-Only one unique dialogue line? Really? Where is her outro interaction with Helena? Why no unique dialog with Lisa, Nico, Ayane, or Eliot?
-Give her a break blow where she beats her opponent's face c'mon KT
-Japanese VA is still great but her new lines are noticeably different sounding than her old ones, from what I've seen the english VA is fine
Conclusion:
Kokoro is stronger than she was in DOA5 but in the context of the DOA6 meta she is weaker overall. Lack of good tracking strings and the strength of sidestep attacking and distance creating tools puts her in a not so great spot. Her throws are still strong and if the opponent lets you get a launcher or fatal stun you can pump out some good damage, but it isn't as safe or reliable compared to other characters. Kokoro feels like a high-risk - moderate reward character. If you guys have any questions or comments let me know, I'm still learning the game so there may be things I am wrong about or haven't discovered yet.
TLDR: Kokoro is a below average character with high risk and moderate reward. Her strikes, holds, and throws all do decent damage but her lack of good tracking moves and gap closers hurts her. DOA6 mechanics make it easier to disengage and put space between players, which nullifies Kokoro's strengths of being an aggressive, in-your-face attacker.