This character is a Other Letter and is mainly used in the Katakana script.
The glyph is not a composition. Its East Asian Width is wide. In bidirectional text it is written from left to right. When changing direction it is not mirrored. This katakana joins with other adjacent katakana to form a word. U+30B9 offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
す, in hiragana or ス in katakana, is one of the Japanese kana, each of which represents one mora. Their shapes come from the kanji 寸 and 須, respectively. Both kana represent the sound [sɯ]. In the Ainu language, the katakana ス can be written as small ㇲ to represent a final s and is used to emphasize the pronunciation of [s] rather than the normal [ɕ] (represented in Ainu as ㇱ).
* スィ and ズィ are also used to present si and zi pronunciations respectively. For example, 'C' is presented as スィー /siː/. See also Hepburn romanization.