You have: Adeleine & Ribbon, Taranza, Hyness, Dark Matter (True Form), Dark Meta Knight, Prince Fluff, Drawcia, Shadow Dedede, Void Termina, and more. There are plenty of viable options for playable characters, each having pretty good merits to the series and, fun fact, none of them were restricted to just one title.
Adeleine has been in Dreamland 3 (1, as Ado), The Crystal Shards (2), and finally back in Star Allies (3). She isn't a one time enemy. She's a character that debuted in 1997 on the SNES and has only made a handful of appearances since...wait a minute... what's that I smell? That's the smell of a character who is almost exactly in the same boat as Geno.
I won't even elaborate on why Dark Matter, Taranza, Hyness, and the others I listed are also of worth to the series. Just because you don't have a tie to these characters doesn't mean they don't have worth because I can assure you that all the ones I listed do to the franchise's whole.
Also, yeah, whoever told you they couldn't bring spirits with them must be blind because Kirby has a literal fountain of untapped enemies from Star Allies alone. Don't even get me started on everything else.