Jeff and Erik fulfill the role of side characters and buddies to Mark Chang in several episodes. Much like their best friend Mark, they speak in a teenage "surfer dude" type slang and act like typical Earth teenagers. For instance, they call their space journey to Earth a "road trip". Not much is known about them individually, other than they love to eat disgusting stuff like radioactive waste, like any other Yugopotamian.
Jeff and Erik once again joined Mark in "Scary Godparents", where they journeyed to Earth at a time most dangerous to Yugopotamians; Halloween, surrounded by lethal candy. When Jeff attempted to handle candy, it burnt through his space suit and hand. Mark and his friends were there to rescue Vicky before the Pumpkinator could destroy Earth, but were forced to flee after Timmy "declared war" on them so that King Gripullon would bombard Earth with candy. While Mark was despondent about having to leave behind Vicky, Erik cheered him up by pointing out he could have a clone of her made. This was the last time Jeff and Erik appeared in the series.
Since the episode "Scary Godparents", they have not appeared again. Mark Chang fled Yugopotamia to hide on Earth from Princess Mandie, he did not bring Jeff and Erik with them and they have not been seen in any more episodes, even those focusing on Mark or the Yugopotamians.