Something you say to someone you become attached to should be said with sincerity. You need to speak with sincerity when you say things to someone who will eventually become your significant other, for example.
When you say something with complete, unabashed sincerity your special saber (せい) starts to pulse with the beating of your heart. It shows the person you want to become close to that you truly are sincere!
Currently there are no similar looking kanji which exist in the WaniKani learning system