I know the timeline doesn't line up since there are only 2 seasons, but Old Man McGucket from Gravity Falls is my pick for this.

In his debut episode he goes from write-off exposition character to mad genius and monster of the week. As he appears more and more throughout season 1, he turns into an effective recurring gag and comic relief.

In S2E7, we learn more about him and the very essence of his character changes. He's elegantly twisted into a tragic character and a core part of the lore over the course of a single episode. Plus, he's emblematic of that episode's themes and moral.

The finale of the show takes what S2E7 did and builds on it. McGucket acts as a core part of the finale's plot and helps the gang face Bill Cipher. He confronts Ford and is finally given closure as well as a happy ending after the final battle. This cements McGucket as a redeemed tragedy — an example of healing and forgiveness in the show that also ties into the themes of not only a single episode, but the entire series as a whole.