Hearts can't be obtained by the number of likes. You need to follow specific events that gives you the chance to gain a character's heart.
In Rhousk's case. Finish "Lizard's Treasure" quest. They will celebrate in the temple after a few days were you'll be invited to celebrate with them. After progressing through the dialogs, Othra will ask you if you like Rhousk. There, you will have three dialog choices:
1.) yes, you like Rhousk.
2.) you like them both.
3.) you prefer her.
Since you want to gain Rhousk's heart the first option would be your choice. I haven't seen the outcome of the other two. But I think you will gain Othra's heart by choosing the 3rd option, or gain both Rhousk's and Othra's heart by choosing the 2nd option.