Ehh...that's not saying much. I really enjoyed the SSE but it really didn't utilize a lot of its cast well at all. As far as characters who even had somewhat of a role...
- Basically the two main protagonists. Fighters who first notice the Subspace stuff and get separated when Petey Piranha and the Primids attacked. Mario's motivation is, like always, Peach's safety first and foremost. Kirby, meanwhile, just wants to help out, considering his friends got kidnapped or are actively missing.
- After watching what happened at the tournament he sets out to help. His motivation seemingly stems from not wanting the tournament interrupted (clearly Pit is a massive sports fan) or seeing lives at stake.
- After the Koopa Army steals their bananas, he and DK set out to stop them, only for DK to be kidnapped. Diddy's motivation is to find DK.
- The Ancient Minister himself. Cowed into submission by Ganondorf and forced to have his people manufacture Subspace bombs, not wanting any harm to befall his race. Eventually stands up for himself after seeing more and more R.O.B.s give their lives, only for his race and the entire Isle of the Ancients to be destroyed in the process.
- Seeks revenge on Wario for capturing Ness, and teams up with the Pokémon Trainer. A scared little kid who was left all alone in the world, he seeks stability. Somewhat completes his arc by blasting off Galleom's arm to help Meta Knight save the Pokémon Trainer.
- He wants to be the very best, like no one ever was. Helps Lucas find Ness seemingly in exchange for helping him catch a Charizard.
- Investigating the Island of the Ancients in an attempt to recover her Power Suit. Also saves Pikachu.
- Was used as an energy source for the Subspace Bomb factory. More of a weapon than a character, but does save Samus from Ridley.
- Had his Arwing shot down. Marooned, he gets literally dragged along by Diddy to help. His Arwing assisted in the fight against the Gunship.
- Trying to recover the Halberd which is being used to spread Shadow Bugs.
- Is aware of everything but due to Terrible People Skills™ he comes off as a bad guy. Develops the brooches that reactivate the fallen trophies.
- Secures an escape route out of the Bomb Factory and assists in the dogfight.
- Serve Master Hand, who is being controlled by Tabuu. All three act as sources of conflict.
- Act as damsels in distress more or less, and are otherwise filler. Ness and Luigi are resurrected by Dedede, but do nothing worth noting on their own.
- Gets the Master Sword from the forest...but we don't know why. Joins Mario after a misunderstanding.
- Is defending a castle from Primid invasions...but we don't know why. Also where the hell is his army.
- Sneaks aboard the Halberd...but we don't know why.
- Show up out of nowhere to help (in Lucario's case after a fight) but have no further narrative purpose.
- Only there for the ride, pretty much. Padding. Olimar at least helps with the Hocotate ship, but still.
- Literally Irrelevant
Then we had Master Hand who was being controlled by Tabuu, and Tabuu himself as the main antagonist.
So in general, a good portion of the cast could have been removed and literally nothing would change. This is most egregious with Link, Marth, and Snake, in my opinion, as they clearly had reasons to be where they were, but those reasons were never given. But yeah, when nearly half the playable roster doesn't have any proper development or even reason for being in the story mode, it's a sign that the cast was too big to handle. Like I said, I love the SSE, but there were some clear problems, and a lot come from cast management.