Toneless 呀 is a phonetic variant of sentence-final 啊 that appears after syllables ending in -a, -o, -e, -i, -ü. After -n, 啊 becomes 哪; after -ng it stays as 啊. Same fundamental particle, different spelling to mirror the vowel that precedes it.
usage
Softens commands (快走呀), warms questions (怎么了呀), and pads agreement (好呀). The 1st-tone reading 呀 is a different word - a standalone interjection and onomatopoeia - and lives on its own page.
Left indexing mouth radical - flags 呀 as a spoken-sound particle, not a lexical word. The 口 radical clusters this with the family of sentence-final and exclamatory mouth-noises: 啊, 哦, 哇, 呢, 吗 - all things said aloud.
Right 牙 supplies the sound: yá loses its tone to become toneless ya as a particle. Pure phonetic borrowing - the "tooth" meaning contributes nothing to 呀. Same phonetic in 芽 (sprout), 讶 (astonished), 雅 (elegant).
