The reformed changeling who becomes king of the changelings is Thorax.
Thorax begins as a changeling who rejects the hive’s traditional practice of stealing love from other creatures. He is friendly and unusually compassionate, and his willingness to share love rather than take it becomes central to his character. His friendship with Spike helps reveal that changelings can survive through mutual care instead of fear and domination.
In the season-six finale, “To Where and Back Again,” Thorax helps Starlight Glimmer and their allies oppose Queen Chrysalis. When the other changelings experience the same transformation, they reject Chrysalis and adopt a more cooperative way of life. Chrysalis is dethroned, and Thorax becomes the new leader and king of the changelings.
The answer is not Ocellus, although she is a reformed changeling and a student at the School of Friendship. Pharynx is Thorax’s brother and initially resists the hive’s new values. Chrysalis is the former queen and antagonist, while Thorax represents the changelings’ reform and reconciliation with Equestria.