Armin is a professional "scare hare," for hire to scare people. He came to Sesamstrasse in episode 2421, where he scared Nils as he was trying to put up a new shelf. Rumpel ended up challenging Armin to a scare duel to find out who is better in scaring others. The competition ended up as a draw.
The rabbit puppet has been used in various roles on Sesamstrasse including Reiner Sauberhasi, Hasi Himmelglück, Hugo Räumlich, Knut Köffelström, Hase Spielhaus, and Henry. He was originally used as Preston Rabbit on Sesame Street.