This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
暒 combines 日 (sun) with 星 (star), but its exact historical origin and meaning are uncertain. It may have been a variant or a character created for a specific context.
The left side is sun 日 and the right side is star 星. Picture a sky where the sun and stars are both visible, but the meaning remains unclear.
For セイ, imagine saying 'say' while pointing at the sun and stars: 'say' -> セイ, even though the kanji's meaning is uncertain.