A 'Jettsen' is a guy who is on one extreme or another. Someone can only ever really like, or really hate, a Jettsen. Jettsens are usually inclined to pursue only one activity, be it athletics, band, choir, etc. A Jettsen is a good friend, if you can get through the long process of getting to know him.
'Did you meet that new kid?'
'Yeah. He started jabbering about video games.'
'Man, what a Jettsen'.