[Heihachirou]

性 is also read as "さが=saga=nature".

This character contains 生(life,born).

So it means inborn traits that you cannot change.

examples,

性別(せいべつ,seibetsu)=sex,gender

性質(せいしつ,seishitsu)=nature,traits

性格(せいかく,seikaku)=personality