Kira hate the mom but in love with the alpha daughter lol. I bet the feeling he had for Selene was the future feeling for his true love Marceline. Remind me of twilight books.
Now at least Kira has told her that truth that's been hurting him in his heart. Soon that sadness will be replaced with the love that Kira well deserves from Marceline; which in turn would make Marceline as happy as he is.