Like the orange soul this is my own personal head-canon, don’t take it as canon.

I think the Kindness kid might have been the only other human who fell down that came close to befriending all the monsters he met, and besides Justice, was the only one to make it to Asgore. But unlike Justice who fought Asgore and lost, the Kindness soul offered the king his soul willingly after seeing how hopeless all the monsters are on his journey. He left his pan and apron behind and asked the guard to take him to the castle.

this one was especially difficult on the old king afterwards...