Oh yeah, you should at the very least watch the story from that game before playing Kingdom Hearts 3.
It's where the main villain is introduced, and his background and motivation is shown, and it's the lead up to the current situation in Kingdom Hearts 3.

I don't know if they'll recap it in KH3, but a lot of crucial plot elements in Kingdom Hearts 3 are the way they are because of what happened in BBS. I'd even go as far as to say that the events of Kingdom Hearts 2 is more skippable than BBS's story, when it comes to enjoying KH3.
Because Kingdom Hearts 2 is more like a conclusion of the first section of the story, wheres BBS introduced a new chapter, that will now conclude in KH3.

Birth By Sleep has my favorite battle mechanics (cooldown system for skills, that can be reduced by using more skills, encouraging more frequent usage of skills) so I'd say it'd be fun to play, if you are at all into Kingdom Hearts. It'll probably pale in comparison to KH 3 though. But if you don't want to play BBS, at least watch the story on youtube is my recommendation.

Anyway, I really didn't want to get my hopes up about this character being playable in the main game, due to what we see in some KH3 trailers. Maybe in a DLC, I thought, but I'm very glad to see I was wrong about that.

First story: Kingdom Hearts 1 > Chain of Memories > 358/2 Days > Kingdom Hearts 2.

Second story: Birth By Sleep >Coded (Only the secret ending, really) > Dream Drop Distance > Kingdom Hearts 3.

Yeah, luckily there are several options for that game. I first played it as an HD Remaster on Kingdom Hearts 2.5 Remix. (PS3)

On PS4 it's on a game called Kingdom Hearts 1.5 + 2.5

And on Kingdom Hearts: The Story So Far

KINGDOM HEARTS HD 1.5+2.5 ReMIX

• KINGDOM HEARTS FINAL MIX
• KINGDOM HEARTS Re:Chain of Memories
• KINGDOM HEARTS 358/2 Days (HD Remastered cinematics)
• KINGDOM HEARTS II FINAL MIX
• KINGDOM HEARTS Birth by Sleep Final MIX
• KINGDOM HEARTS Re:coded (HD Remastered cinematics)

KINGDOM HEARTS HD 2.8 FINAL CHAPTER PROLOGUE

• KINGDOM HEARTS Dream Drop Distance HD
• KINGDOM HEARTS 0.2 Birth by Sleep –A fragmentary passage–
• KINGDOM HEARTS χ Back Cover (movie)

It basically has every game. Including Dream Drop Distance, and 0.2 (An epilogue to BBS, and Prologue to KH3 featuring Aqua).
^This one seems to be currently sold out on Amazon, though.

I wouldn't even consider Birth By Sleep a spin-off to be honest since it has both the core gameplay and story. It just features a different main cast, but that's some times necessary, like how Metal Gear Solid 3 is about Snake's father, etc.
I think they should have given it a number, like KH 0. The short epilogue to Birth By Sleep is even called KH 0.2.

Nice.
Yeah from what I understand (I haven't played it myself, but I watched a story summary) Coded is mainly a side story, but the secret ending reveals something regarding the main villain of the current story, and sets up for the events of Dream Drop Distance, which is where Riku and Sora go to take the Mark of Mastery, to try to get the keyblade 'Master' title, in preparation for the final battle to come in KH3.
As of Kingdom Hearts 3 there are currently four people with that title. Aqua is one of them.

I forgot to mention that Kingdom Hearts X also plays a part in the KH3 story, and it's summarized in video form in Kingdom Hearts: The Story So Far, or Kingdom Hearts HD 2.8 Final Chapter Prologue.

I don't mention the character by name, but in the opening post I provide a clickable link where you can see a cropped out image of the character if you click it.
The other images I've posted in other replies are unrelated to the topic, but rather talking about other KH games and whether they are worth playing, etc.

Others in this thread have avoided mentioning the character by name as well, though some comments have been made that could be seen as a hint, so I would not recommend anyone read this topic unless they are ok with knowing who the playable character is.

I posted this because I know there are people who want to know about playable characters, even if they don't want to hear anything else about the game.
Since Kingdom Hearts games essentially always have multiple playable characters (even in KH1, you get to play as a Heartless, for example) the headline itself shouldn't be a problem.

Yeah, that one sounds like the best deal.
I have played X actually. If it's worth playing? Well, it's a mobile gacha game, and it plays completely different from any other KH game. It's not something I'd recommend someone to play for the story, but rather if you happen to enjoy things like collecting your favorite characters, and the gameplay mechanics of the game.

You use in-game currency to get random characters and hope you get lucky to get the ones you want. Though the characters are usually guaranteed after a certain number of tries, which makes it possible for people to play and get most of the characters they want even without spending real money on the game (which I don't).

It can be nice for killing a few minutes here and there, and there are some interesting strategies for completing some fights in the game.

That said, my interest in it has waned a lot, to the point where I now just log in for the daily log in bonus (in-game currency), and save them for when a character I'm interested in shows up. I don't really play it any more. But that could change if they add some interesting new modes.
It's free to download, so you could always give it a try and see if it's something you'd want to keep playing for a while.

The story may be worth knowing though, whether you get it from playing the game or looking at the videos featured in the PS4 game KH:TSSF. I say 'may be' because they may summarize the important bits in KH3.

One thing to note though is that KHUX's story, including the part about the Org XIII member I mentioned, still hasn't concluded. (They add more story every now and then through updates.) Which is interesting, since KH3 will be out in a few days. So I guess I'll get most of my answers about these characters in KH3 first?

Yeah, the 'gacha' (gambling for rare units) element of it is the same as in Fire Emblem Heroes. Though Heroes at least plays similar to the real Fire Emblem.
It's nice that you plan to catch up on the full story before KH3. I plan to do the same, sort of. I have Coded and Dream Drop Distance left, but I think I'll skip playing Coded, and just potentially watch the story in video form. I already know the jist of DDD's story since I looked it up when I didn't think I'd play it, but since it got an HD Remaster, I'll give it a go before KH3 comes out. I have a 3DS but usually I'm not keen on using it for 3D games due to the analog and lack of camera control options.

Depending on how long it takes you to get through these games, I hope you're successful in avoiding seeing any KH3 related spoilers you're not interested in learning before playing the game, because that can be a challenge. Everyone in this topic has been responsible so far and didn't even mention the character by name even without me asking them to do so, but you never know when someone might show up and blurt something out.

Though I managed to avoid Metal Gear Solid 4 spoilers for like 5 years, so a few days/weeks for KH3 shouldn't be a problem.

Yeah, someone else did the same, and that was nice to see. Even though I figured people clicking on this topic will primarily be those who are ok with finding out the identity of the character, there's a chance that someone may not be. And so far, one person wasn't. So I'm glad other people were thinking the same way.
Speaking of which, I'm currently asking him what exactly he finds problematic with my topic, but he did essentially say that my topic wouldn't be helpful to anyone because people can just look this up on other sites.
But given what you just said, you seem to be exactly the the type of person this topic was aimed towards.

Is it fair to say that you'd worry about accidentally seeing other spoilers you wouldn't want to see, if you were to venture into some Kingdom Hearts community site that's full of spoilers, or a Youtube channel posting a bunch of footage from a leaked copy of the game?
Because that was the case for me. I wouldn't actually try to dig this info up in those places because it's like going through a minefield. I happened to come by this info by chance, and took a look to see if it really was what it seemed to be, and then I thought I'd try to help some other like-minded people who may be interested in this particular info, but would want to avoid most other story related info, by making a topic that appears to have little risk in getting anything else spoiled in the process.

Even by doing that, I get that some people who may be curious to see if the character mentioned in the topic title is who they are hoping for, are still worried that other things would be spoiled in the process, and decide not to read the thread. Very understandable. But at least a few people 'took that chance' seemingly, and if it helped them by avoiding other spoilers, that's exactly why I made the topic.

Well that and if they wanted to share their thoughts on it, I wanted to hear that as well.
You for example said something that surprised me a bit, which I thought was interesting. That you were looking forward to this character even though you hadn't played certain games in the series. Which may be a testament to characters design, if people are drawn to them before getting to know them. Some times I see a character and immediately feel 'I'm probably going to like this character'. Recently that was the case for me in Persona 5 with Joker and Queen, just from watching trailers.

From what I've read about Coded's story, it did sound almost like an ambitious recap of KH1, with elements and characters of other games added as well, so I'm not surprised to hear you describe it that way.
I did check out the secret ending on Youtube though, since I heard it's significant to the main plot. And about that, since you said you tend to play these on their original system, apparently the secret ending to Coded is exclusive to the HD 2.5 ReMIX cinematic version. So if your only experience with Coded is the DS version, you may want to look up the secret ending on Youtube.
Though since you haven't played BBS yet (and I take it you haven't played DDD either?) it's probably best if you check out Coded's secret ending after you finish BBS, because it revolves around the main villain's plan, after the events of BBS and leading into DDD.

I didn't pick up Smash yet, but I also need to finish Persona 5 before KH3 comes out, so there is that as well. But KH3's impending release has motivated me to get through my backlog finally, which I started doing last week!