Like Kendra, he has this really strange knack for managing to stay alive in the trickiest situations, even though he is often mortally wounded, and never gives up.
Seth successfully retrieves the Chronometer and inadvertently brings Patton Burgess to the present time.
Seth gets into more trouble by stealing the first horn of a unicorn from the centaurs, their most prized possession (aka the soul of Grunhold/Ring of Stones).
He is then denied from the group journeying to the dragon sanctuary Wyrmroost because of his age, (and his possible dark attributes) but that won't stop him from going.
In a plot to betray the Sphinx, Mirav following instructions from Nani Luna secretly gives Seth The Translocator and The Sands of Sanctity.
Unaware of the plot Seth quickly leaves using The Translocator with The Sands of Sanctity and travels back to Fablehaven.
The message contains Patton's plan for Seth to arm himself and get to the Shoreless Isle to make a last stand against the demons.
Along the way Seth barters with the Singing Sisters alone and promises to give a wraith, give Vasilis one year after the recovering it, and a promise to complete on impossible task in the future in exchange the information and if he fails to do any of these tasks he will be killed by a magic knife.
Seth is a Shadow Charmer, his qualities beginning to emerge in Rise of the Evening Star and being stabilized and brought to full potency in Secrets of the Dragon Sanctuary.
It seems to be that in their opinions, Seth's accomplishments weren't as significant as Kendra's, perhaps because of the dark elements that always surrounded them.