surname
凊 is a rare kanji used primarily as a surname. Its standalone meaning is unclear, but it is associated with coldness or coolness through its readings.
凊 combines the ice radical 冫 with 青, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
Ice 冫 beside 青 (blue/green) suggests a cold, clear scene. Think of a surname written with the image of icy blueness.
For セイ, imagine a person with the surname 凊 saying their name sounds like 'say': セイ -> 'say'. The ice-cold blue character matches the crisp sound.