Roman numeral code: vii
Scale notes: 7, 2, 4
Attributes: finality, resolution
7 is all but unknown in popular music, and is rare even in classical music.
7 can function in a similar way as 5, but does not have the strength of 5. Its main role is to resolve to 1 with a different feeling from the usual 5 -> 1 cadence.
7 does not contain scale note 5, and doesn't offer the same sense of support and validation that 5 does. It has very little power itself, and can generate energy only in the 7 -> 1 transition. Even though it is a diminished chord, 7 in context seems to lack the characteristic diminished sound. We hear it just as a harmony that wants to immediately resolve to 1.