Seimei doesn't have a canonical romance. I made up one for his Millicanon backstory - specifically, with his second wife, Minamoto no Tsuyu (nicknamed Suzume).

YT canonically has an unrequited crush on one of her canon mates and a brief and very ill-advised fling with another one. Both of these while she had a formal boyfriend, who she did not seem terribly emotionally attached to. I'm not sure that she is, or ever will be, capable of a close, monogamous romantic relationship.