To start off, Asgore is already hinted to be important in Chapter 5, with a boss that's related to him, the theme of flowers, and more. But, I'd like to suggest Asgore has more connection to the Roaring Knight as well, but first lets see some story for this character.
To begin, Asgore's shop/house is only open in the first chapter, and in Chapter 4, it reveals that there's a basement with a corkboard and documents.
It is also said at the Police Station that Asgore was removed from the police force as a chief for an unknown reason.
And in the church section of Chapter 4, if you talk to Alvin about Asgore, he mentions Asgore coming to church at night to pray.
He also works as a cleaner for the Holiday residence, and during when your SOUL goes through the vents, you can find Asgore by a corkboard, discussing about an "old friend", "just wanted to protect everyone", and also mentions a black shard, a weapon you get after defeating the Knight in Chapter 3.
One day when I was playing Chapter 4, the part where we discuss about the clues in the church, Susie mentions, "Someone who goes to church... who knows the mayor... Who also has something to do with the cops?"
And that's where it clicked, Asgore.
Think about it, he goes to church at night, he works as a cleaner for Carol's house, and used to be the chief of the police force. If anyone knew how to unlock the shelter, it would be him. Not to mention, that Asgore is most likely the only known person in Hometown that's stronger the Undyne, explaining how someone carried Undyne from the top of the town to the Shelter at the bottom.
I would conclude that Asgore could be the Knight, but there's issues.
First off, Asgore is simply too fat to be the Knight's skinny body, unless if the dark would could change the anatomy of a lightner, but there's no case of this.
Secondly, Asgore would be too busy to attend to the Dark Sanctuaries as he's probably still cleaning the Holiday residence.
Which is why instead, I think that Asgore could be affiliated with the Knight, a second knight if you will. I would assume that the Knight is the "old friend" Asgore discussed in Chapter 4.
During the end of Chapter 3, the Knight could've been called to the Dreemurr residence to kidnap Undyne because she could also know about the shelter code, and Asgore could've assisted the Knight by carrying Undyne down to the Shelter.
During a scene in Chapter 4, right before Kris gives us back control, Kris calls supposedly the Knight about Susie having the guitar, and the caller says, "I'll be... right there...", besides Carol entering the house, Asgore also comes in the living room, interrupting Susie and Noelle's interactions and tells Noelle about her mother coming in.
I'm less sure of this, but Asgore could've called Carol about the things happening in the home.
And after Susie and Kris leave the house, when Susie mentions forgetting about the code, Asgore comes in to say not to worry about things.
I believe that this could be more supported by the time Chapter 5 arrives, as other theories, like ROOTS theory theorize that Asgore could create Chapter 5's dark world, being a second knight could support this.
And if the Knights' kidnapped Undyne to avoid the possibility of someone else opening the Shelter, they could kidnap others like Carol, and Alvin following the same logic. Though this probably isn't the case.
For now, this is all I've got for this theory, but feel free to discuss anything to support or be against this theory.
