You and Riku are best friends, you seem to take a notice of him getting very jealous when it comes to you spending time with your 'new' friends. You catch him glaring and staring a couples times as he thinks you have abandoned him, maybe reassuring him would help?...Nope...He still believes that you abandoned him.

his jealousy would grow even more as one night you two were sitting by the beach in a treehouse, he stares at you as he scoots closer to you and sits next to you. He now whispers in a tone that was laced with jealousy, "How are you doing..With your new friends.." He still seemed super over protective and jealous of you.

Maybe he just has abandonment issues and is very scared of you leaving him in the dust for new friends? He now holds your hand tightly as he stares at you.