The kanji 正 originally depicted a foot (止) stopping at a specific point or line (一). This symbolizes 'stopping at the correct place' or 'standing upright and straight'.
'Sei' — like 'SAY' the correct answer when you're sure it's RIGHT.
Imagine a perfect square, showing something that is 'correct' or 'regular'. The top line is 'one' (一), and the rest is like an upright stop sign (止) - stopping at the right place.