In the First Assassin's Creed Game, you begin at the Maximum Level. Max Synchronization, all weapons, and all skills. Then Altair foolishly broke all of the Assassin Tenants and was demoted to Novice, having all his equipment taken away and wasn't even allowed to jump and grab ledges unless his master said it was okay. Nice job Altair.

Parker was an elite in the Pelenka Mercenary Corp. However, he soon broke the Rules of non-interference (He broke up a peace Treaty between the Pull Up wearers and the Underwear Clan) and was being punished. He was stripped of his pelenka and dressed in a disposable diaper that is worn by Novice Soldiers until they earn their pelenkas. Parker hates disposables because they make too much noise.

The Diaper Parker is wearing is some kind of Japanese Pull up or diaper. You can thank Landonbay for introducing me to it.