U+1D97 Latin Small Letter Open O with Retroflex Hook

The Wikipedia has the following information about this codepoint:

An r-colored or rhotic vowel (also called a retroflex vowel, vocalic r, or a rhotacized vowel) is a vowel that is modified in a way that results in a lowering in frequency of the third formant. R-colored vowels can be articulated in various ways: the tip or blade of the tongue may be turned up during at least part of the articulation of the vowel (a retroflex articulation) or the back of the tongue may be bunched. In addition, the vocal tract may often be constricted in the region of the epiglottis.

R-colored vowels are exceedingly rare, occurring in less than one percent of all languages. However, they occur in two of the most widely spoken languages: North American English and Mandarin Chinese. In North American English, they are found in words such as dollar, butter, third, color, and nurse. They also occur in Canadian French, some varieties of Portuguese, some Jutlandic dialects of Danish, and in a few indigenous languages of the Americas and of Asia, including Serrano and Yurok in the United States, Luobohe Miao in China, and Badaga in India.