realize
惺 centers on a mental awakening: coming to understand or realize something clearly.
惺 combines the heart radical 忄 with 星, which likely contributes the sound and suggests a bright, star-like clarity of mind.
The heart 忄 beside a star 星: picture a sudden star lighting up in your heart, and you realize the truth.
For セイ, imagine a sage (聖, せい) whose heart is lit by a star, helping you realize deep wisdom: sage -> セイ.