The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it is written from left to right. When changing direction it is not mirrored. The word that U+0259 forms with similar adjacent characters prevents a line break inside it. The glyph can be confused with one other glyph.
The Wikipedia has the following information about this codepoint:
Ə, or ə, also called schwa, is an additional letter of the Latin alphabet. In the International Phonetic Alphabet (IPA), minuscule ə is used to represent the mid central vowel or a schwa.
It was invented by Johann Andreas Schmeller for the reduced vowel at the end of some German words and first used in his 1820s works on the Bavarian dialects.