clear
晟 centers on brightness and clarity, like the sun shining clearly. Its meaning of 'clear' extends naturally to things that are bright, evident, or unobscured.
晟 combines 日 (sun) with 成, which likely contributes the sound and the idea of completion or fullness, suggesting the sun at its brightest. The exact historical development is uncertain.
The sun 日 sits above 成, like the sun reaching its peak brightness. When the sun is fully out, everything becomes clear and bright.
For セイ, picture the clear sun saying 'say' hello to a bright day: say -> セイ, and the sun makes everything clear.