Diana's family has the blessing of the moon, so Cassius is a moon/star child. The blessings are sort of like magic but as the generations went on it became a much more subtle, very little known thing even to those who process them. In their case it's like feeling safer or healthier during moonlit nights, especially the full moon and they have a guardian deity, Celeste. When Diana dies/vanishes, she watches over Cas, but he knows nothing of any of this.
Since Kuno is essentially his foil, he's like sunshine. He doesn't have any blessing and is otherwise not special, but to Cas he shines so brightly.
And as we know, the moon since because of the aligning with the sun. So while the sun is brighter and bolder than the moon, he isn't stronger, he makes Cassius stronger and want to be better. Kuno is loud and okay with being the center of attention as a "class clown" but he's only happy with it if he can get Cassius to laugh and cheer up.
