exclamation
吁 centers on a vocal outburst: an exclamation, sigh, or cry. It represents the act of making a sudden expressive sound.
吁 combines 口 (mouth) with 于, which likely contributes a phonetic element. The character expresses a vocal sound or exclamation.
The mouth 口 on the left and the shape 于 on the right suggest a sudden sound escaping the mouth: an exclamation.
For ク, imagine a person exclaiming 'Cool!' with a sudden outburst: Cool -> ク, and the mouth 口 releases the sound.