I'd go with either
Free Time
time available for hobbies and other activities that you enjoy Free Dictonary
another definition
Free time is time spent away from business, work, job hunting, domestic chores and education. Wikipedia
Or if wanting more formal then:
Lesuire time
time available for ease and relaxation; "his job left him little leisure" Free Dictonary 2
I'll go with downtime
time spent not working; free or leisure time
Webster's New World College Dictionary
I'm looking for a word to describe the person you prefer to spend most of your time with who could be anyone from a relative to a lover. They’re not just a “companion”, they’re your “chosen companion” among many, if that makes sense. You have other people in your life that you care about, but you always choose to be with this person over everyone else, you always find yourself gravitating or coming back to them, specifically when "spending leisure time together with someone, doing any kind of activity", they're always your No.1 choice.
What can we call that person?
Note: this was also posted on r/whatstheword