Koga in Japanese means "old river" when used with the kanji of (古河) or "ancient joy" when used with the kanji (古賀).
In Japanese Koga has varied meanings depending on the kanji that is used, but most commonly Koga in Japanese refers to a place name like Koga city in Ibaraki, or even as a surname, which often means old river or ancient joy.
Koga in Japanese is a historical name that is also linked to samurai, with different kanji combinations giving the name Koga meanings like "long time self" when used with the kanji (久我) or even related to stealth.
And some fictional characters use kanji for "bright fang" (晃牙) or link to "wolf" (大神).
The historical and fictional contexts of Koga in Japanese include.
Samurai Clans in which the Koga name has ties to samurai, potentially meaning "to hide/conceal," referencing stealth.
Fictional Characters, in Ensemble Stars, the character Koga Ogami with the Kanji of (大神 晃牙) has a name that means "Great Deity" but puns on "wolf" (ōkami), fitting his wild personality.
The common meanings and Kanji combinations of Koga include.
古河 (こが - Koga) which means "Old river," a place name.
古賀 (こが - Koga) which means "Old celebration/joy," also a place and surname, common in Kyushu.
久我 (Kuga/Koga) which means "Long time/self," used by an ancient aristocratic family (Minamoto clan).
古画 (こが - Koga) which means "Ancient painting".
Koga is not one single word, instead it's a sound with multiple written forms of the word and meanings, that is usually tied to places, families as well as descriptive traits.