It represents the phoneme , although for phonological reasons (general scheme for /h/ group, whose only phonologic survivor to /f/ ([ɸ]) remaining is ふ:   b<-p<--f-->h), the actual pronunciation is , which is why it is romanized fu in Hepburn romanization instead of hu as in Nihon-shiki and Kunrei-shiki rōmaji (Korean 후 /hu/ creates the same phonetic effect as lips are projected when pronouncing "u"). Written with a dakuten (ぶ, ブ), they both represent a "bu" sound, and written with handakuten (ぷ, プ) they both represent a "pu" sound.
It represents the phoneme , though due to phonological reasons (general scheme for /h/ group, where only ふ:   b<-p<--f-->h remains as [ɸ]), the actual pronunciation is . Thus, it is romanized fu in Hepburn instead of hu in Nihon-shiki and Kunrei-shiki rōmaji. Written with dakuten (ぶ, ブ) or handakuten (ぷ, プ), they both represent a "bu" sound.