What's Up With Chisei?

One of the most commonly misunderstood, misinterpreted, or just plain missed things in the canon is the question of what, exactly, is going on with Chisei.

Which is to say: why is his age in quotes, is he being serious when he said he died, and what is going on?

The short answer is:
Kuzuryu Chisei is dead, and the Chisei that we see interacting with characters in the present in the canon is a phantom illusion being manifested by Haruomi.

What is going on with Buraikan?

Chisei used to be a professional basketball player before he started doing music. Haruomi seems to have inherited running a Chinese restaurant from his family. Haruomi apparently did or was interested in music but was about to quit doing it following his father's suicide. He met Chisei and they founded Buraikan 14 years ago, and they obtained their phantometals 11 years ago. They popularized phantom lives.
[source: the Buraikan is Back feature, Allen's infodump in MEMORY, character timelines from the second fanbook.]
Approximately ten years ago, Chisei collapsed on stage because of phantometal erosion and Haruomi did a metal kiss mind-dive to save him. This is apparently not well documented; Iori talks about it as being somewhat apocryphal, and even Allen, Buraikan mega fanboy that he is, has never heard about it. (The fact that this is an urban legend, and that nobody seems to know why Buraikan disappeared, may indicate that this was not as successful as Iori claims.)
[source: VIBES Part 2]
CLUB Paradox's re-opening and the announcement of the Paradox Live starts the series, ten years after Buraikan's sudden disappearance. Buraikan have not been seen at all in the intervening ten years, so people are super skeptical of the idea that Buraikan are actually involved with this at all, even as we get to the end of Round 1.
[source: Opening Show tracks and the skepticism is again referenced in the Buraikan is Back feature]
Their first major in-person appearance, together, is in LIVE pt.4: Haruomi is revealed to be the old man who runs Raimentei, appearing much older than he should. Chisei, meanwhile, appears to have not aged a day in ten years when he appears.
Haruomi seems surprised to see Chisei, who says that it's "thanks to [cozmez] that I'm able to come out like this", and remarks that Haruomi has become an old ramen-shop owner "because I up and died and left you behind". (You'd think that this would be pretty incontrovertible proof, but it's a very throwaway line that nobody really dwells on, and a lot of people found it hard to tell if Chisei is being facetious.)
[source: LIVE Part 4]

Additional Evidence

One piece of official merchandise (seen here) gives Chisei's canonical age as 28. If assumed that this is his current age, then this would have made him at the absolute oldest 17 when Buraikan was founded, which does not leave a lot of time for him to have been at the peak of a professional basketball career before picking up music.
Though subtle, Chisei never has a reflection in any MVs where he appears, most notably in Rap Guerrila Reload and BURAIKAN is Back.

Furthermore, all of the the visuals in BURAIKAN is Back only seem to really draw from Haruomi's phantom illusion and not Chisei's, as seen in their Phantom trailers from the third anniversary (Chisei's uses lightning and gold sand, where Haruomi's is black liquid and smoke). This is somewhat muddied by future MVs being a bit more egalitarian, but was still an early piece of the puzzle.

So what does that mean?

With the above evidence that Chisei is dead, the general working understanding is that the Chisei we see interacting with the rest of the cast is a metal phantom created by Haruomi -- inspired by Kanata doing the same to create a phantom Nayuta -- based on Chisei as he was ten years ago at age 28. Hence, his age is in quotation marks.
Haruomi's aged appearance in the present is loosely implied to be the result of premature aging thanks to habitual phantometal use, and it is possible that his own younger appearance is also a phantom created for the purpose of public appearances/performances.

When Chisei appears in LIVE part 4, he is assumed to be an illusion. Haruomi, unlike Kanata, appears to be aware that he is manifesing an illusion of a dead person, but is keeping the illusion up for his own undisclosed reasons.

We know from phantom Nayuta that metal phantoms are capable of acting and speaking indepenently from the person generating them, and are corporeal enough to manipulate physical objects (see: the Nayuta phantom in the LOVE dramas).

So, yes, when Chisei says "because I up and died", you should take it at face value.

But that said...

... so I wrote all this a while ago, and now this is all kind of a moot point, because the anime basically outright said that Chisei is dead.
So, hey, I was right, at least!

Whether you take the anime as canon or not is your own cross to bear. Many parts of the anime, especially (unfortunately) the parts from which we draw this information, are incompatible with the drama track canon, but if you choose to accept it as canon, then it resolves this ambiguity entirely.

The anime features a phantom Buraikan created by Alter Trigger for the purposes of the first Paradox Live; if we attempt to fold this information into the drama canon, we can assume that Haruomi continues to create the phantom Chisei on his own. Indeed, the special voice dramas included as a bonus from the full BD set confirmed that Haruomi was specifically inspired by Kanata's use of phantoms to create a Chisei phantom.

Visuals at the beginning of episode 11 seem to confirm that it was phantometal erosion that killed him. A line from the Alter Trigger scientist may seem to contradict this, thanks to the way the official subtitles say "the same way Yasha died" as he's firing a gun at Shura; but, based on the actual line in Japanese, I believe this is probably meant to mean "and you're going to die, too", and is not a remark on how Chisei died.

Again: unfortunately, there are many things about the anime that are incompatible with canon, but the Buraikan information does mostly track with and confirm what was hinted at in the drama tracks, because Buraikan are basically the only group that made it out of the anime without their story and characterization being put through a meat grinder, lol.