Hangul syllable swelg

It is one Hangul syllable; the word around it supplies its role. Hangul’s component letters, called jamo, group into square syllable blocks. This block combines ㅅ as its initial consonant, ㅞ as the vowel, and ㄺ as its final consonant cluster, called batchim.

Hangul’s history predates this particular block. In 1443, King Sejong of Joseon, the Korean kingdom of the time, announced 28 letters for writing Korean; the first record names the new script Hunminjeongeum. A book bearing that name was published in 1446 to explain the letters, their principles, and how syllables are built. 쉙 is one assembled syllable in that system, not a separately invented emblem.

A lone 쉙 has no reliable tone

Read it as ordinary Korean text until the surrounding message adds meaning. In a Korean sentence, adjacent syllables form the word; a single 쉙 does not tell you whether the sender is joking, affectionate, or annoyed. Ask what it refers to instead of assigning a mood.

Font-dependent. A font can draw the Hangul block with a different outline; that changes how it looks, not which character it is.