What he did in the series
In The Lure Depths, he was seduced by a large green amphibious creature in the depths, signaling to eat the worm. Guppie was stupid enough to fall for the trap because "sometimes pretty things are attractive" and was skewered on a meathook, where he then slowly suffered from blood loss and was turned into deepfried fish filet turned into tiny flayed cubes of meat, just as he had envisioned in his worst nightmares. Worst of all? He was alive when they began to eat him.
Fishtales and beyond:
The fish in the last three episodes is not him, contrary to popular belief. Instead, it is an corrupted soul who created his proxy body using Guppie's remains. Even though he had a lot of character development and dialogue, he didn't really do that much in the series in the last 3 episodes.