The character こ with a handakuten.
こ゚ (romaji ngo)
- (nonstandard) The hiragana syllable こ゚ (ngo). It's equivalent in katakana is コ゚ (ngo).
[ŋ] is an allophonic variation of /ɡ/ in Japanese; this phenomenon is known as 鼻濁音 (bidakuon, “nasal voiced sound”). This character is not used in standard Japanese, but instead for distinction between the two allophonic variations by Japanese linguists.