Left indexing mouth radical — marks
呦 as a vocalization. Anchors it among interjections and animal-sound characters alongside
哎 (sigh),
哦 (oh),
啊 (ah),
哇 (wow).
呦 was originally the cry of a deer (
呦呦鹿鸣 "yōu-yōu the deer cries"), now extended to a soft human exclamation.