Depends on the timeline. Nnoitra is one of the older Arrancars; he was in Las Noches already during the time Baraggan ruled HM. Remember, he made that comment about how he no longer needed to be respectful to Szayelaporro once he was no longer an Espada. He was #8 when Szayelaporro was Cero. I guess the question is, was everyone under Baraggan's rule a Hollow -- or did some manage to Arrancarize themselves already? We could make the argument that Baraggan with his axe was an Arrancar who preferred that form instead of a human form; we do not see Hollows wielding a separate weapon normally -- or wearing clothes.
We also have three other figures on the dais with Baraggan who are also wearing clothes:
The page with the ewer, the tall figure with the white cape (my crack theory is this is Nnoitra since it looks like the collar of his uniform), and a third figure with a heavy cape. Two of Baraggan's Espada, perhaps? So I believe there were some Arrancar under Baraggan already before Aizen arrived.
Hopefully we'll get an answer since Isshin definitely knows about the "mock-Arrancar" that were in HM a while back. I would like to see that information included in this flashback.
We do know about the rain falling when he appears -- and Kubo has been hammering that symbolism in since we found out about how Masaki supposedly died. I would like to think there's some connection there, that there's a reason Nnoitra got Despair instead of one of the other Aspects. It's not like Kubo has expounded at great length about Old Age or Madness in the manga.
You don't think Nnoitra is a bit emo with his hair over one eye?
He's like Kira in that respect, and there is that in-manga character comment about Kira being emo from his FKT battle. Nnoitra is cuter though.
I hope he's still alive too. I mean, if Kubo can confirm Gin dead, I would think he would have said something to confirm Nnoitra by this point if that were the case. He seems to dodge on certain characters, especially those he likes. He still hasn't explained what happened to Nnoitra's zanpakutou that it changed; that may be relevant considering Harribel's girls managed to get a power-up as Arrancars. If they did much the same feat as Nnoitra, it should be explained in-story even if the cause of their power-up is obvious.
I would make a correction though about being tied to Nel. Much like the situations of Ichigo fighting Grand Fisher and Byakuya, Nnoitra had to fight to work out his issue with Nelliel, and that issue is IMO related to Zaraki holding himself back in hopes of fighting Unohana again one day. Once his battle with Nelliel ended, he like Zaraki could finally move forward for a higher calling in battle. Zaraki finally communicates with his zanpakutou as a true Shinigami, and Nnoitra ... well, we don't know.
He was supposed to figure out why he should be fighting. I doubt Kubo dropped the thread entirely, he never does. He will pick it up at the proper time. It probably would have ruined something to show at that point where Nnoitra's heart is; not yet enough build-up or general humanizing of the Arrancar species. We didn't really get that information for
any Arrancar until Ulquiorra turned to ash forty chapters later.
Also for Nel's part, the one for whom she regained adult form last time was Ichigo -- with intent to kill Nnoitra for hurting Ichigo. It is Ichigo who gives her the impetus to wield her old powers for a short time; it is for him that she (temporarily) regained her heart and will to protect. She is happier not fighting, according to her brothers, but would certainly run to Ichigo's rescue if needed. In Kubo's pattern though, Nnoitra would probably never fight Ichigo again, so there's no reason for Nel to need her old powers to fight Nnoitra again.