A lot can be said about the multiple facets of this game's gameplay and whatever but personally the thing that's the most interesting thing about the game is its structure. As someone who played it on hard, Sonic Frontiers is an easy game with an easy way out.
There are multiple different collectibles that unlock certain things and unlocking those things is generally how you'll progress the story. Collecting portal gates from minibosses in the hubworld unlock the platforming levels. Completing challenges in the platforming levels gives you vault keys that unlock the vaults holding the chaos emeralds needed to complete the story. The final main collectible is memory fragments that are found generally by completing the small platforming challenges scattered about and they are also used to progress the story. Nothing is really finite so you'll gain some of these by chance from doing shit like cylooping the ground for fun.
However, the real other way to gain all this shit is by fishing. The throne things that usually send you to one of the platforming challenges can send you to the fishing world where Big the Cat is for some reason. If you collect random purple coins scattered about (or get them from the random slot machine thing that pops up during the random meteor shower event) you can catch random things, gain tokens that way, and spend all your tokens on literally any collectible the game has to offer. If you get lucky enough to get a few hundred purple coins from the meteor showers and make it to the fourth world you can bypass pretty much the entire fourth world by paying the Big toll and it's pretty mental. It's also the only way to efficiently max out your levels because you can buy like 4000 koco and sit in front of the elder for 30 minutes mashing through slow menus upgrading shit to level 99 one level at a god damn time.
TL;DR Big the Cat is the most important character in the game
There are multiple different collectibles that unlock certain things and unlocking those things is generally how you'll progress the story. Collecting portal gates from minibosses in the hubworld unlock the platforming levels. Completing challenges in the platforming levels gives you vault keys that unlock the vaults holding the chaos emeralds needed to complete the story. The final main collectible is memory fragments that are found generally by completing the small platforming challenges scattered about and they are also used to progress the story. Nothing is really finite so you'll gain some of these by chance from doing shit like cylooping the ground for fun.
However, the real other way to gain all this shit is by fishing. The throne things that usually send you to one of the platforming challenges can send you to the fishing world where Big the Cat is for some reason. If you collect random purple coins scattered about (or get them from the random slot machine thing that pops up during the random meteor shower event) you can catch random things, gain tokens that way, and spend all your tokens on literally any collectible the game has to offer. If you get lucky enough to get a few hundred purple coins from the meteor showers and make it to the fourth world you can bypass pretty much the entire fourth world by paying the Big toll and it's pretty mental. It's also the only way to efficiently max out your levels because you can buy like 4000 koco and sit in front of the elder for 30 minutes mashing through slow menus upgrading shit to level 99 one level at a god damn time.
TL;DR Big the Cat is the most important character in the game