亥 holds nine to eleven at night, the last of the twelve, and in the year the start of winter, where water takes over and everything returns to being hidden. It's yang water: the great river, the sea.
As the end of the cycle, its function is return and absorption: everything that travelled dissolves here, and only then does it come round to 子 and begin again. So it carries both tolerance and ending, which isn't a contradiction — whoever can contain is who can close. The pig is the personality version of that position.
Where 亥 sits in a chart, that area is about release and endings, and you're usually better than most at accepting that something is simply over. 亥 is also a travelling horse and opposes 巳: summer's concentrated fire against winter's spread water. The tension usually shows up as whether to tighten or to let go.
