This character is a Other Letter and is mainly used in the Katakana script. The character is also known as SMALL TSU.
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+30C3 prohibits line breaks before its position. There are some exceptions, though.
The Wikipedia has the following information about this codepoint:
The sokuon (促音) is a Japanese symbol in the form of a small hiragana or katakana tsu. In less formal language, it is called chiisai tsu (小さいつ) or chiisana tsu (小さなつ), meaning "small tsu". It serves multiple purposes in Japanese writing.