Why is it der Kasper?

Kasper ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] particularly funny main character of a puppet theater that is named after him

[1] besonders lustige Hauptfigur eines Puppentheaters, das nach ihm benannt ist

[2] Slightly pejorative: person who is perceived as silly

[2] leicht abwertend: Person, die als zu albern empfunden wird

Examples

[1] "The Kasper looked different from our"

[1] „Der Kasper sah anders aus als unserer.“

Declension

1 Singular Plural
Nominativ der Kasper die Kasper
Genitiv des Kaspers der Kasper
Dativ dem Kasper den Kaspern
Akkusativ den Kasper die Kasper

Dictionary entries include material from Wiktionary. Wiktionary