Family Name, Surname
If there's a special woman in your life, she might eventually take your family name and make it her surname too. In Japan and other countries, tradition dictates that women change their last name when they get married and keep that name for the rest of their life. Their partner’s family name becomes their family name too.
Imagine having your surname changed for the rest of your life. How do you feel about having a different family name? Is it a big deal to you or does it not matter much?
せい, しょう
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Picture this special woman in your mind. She's changing her surname to yours and taking hold of your special saber (せい) as well. And it's... it's splitting in two!
Now you share a family name AND a holy blue saber! You can protect your legacy (and surname) hand in hand and saber in saber. (Or something like that.)