Me again, back to theorizing Rose Quartz! This one more geared towards Lion's relationship to Rose and fantasy magical practices! And a certain context in an episode. Lion is Rose's familiar. Familiars (as noted by a book series in SU) are invisible to all but the "owner" Thus when Rose was still present to maintain the master/familiar relationship no one could see Lion. BUT, when Steven got close with Rose's gem, Lion attached, and since Steven is unskilled, Lion is now visible! - WIP Theory.
Hmmm, interesting! I sort of like the idea of Lion being present for Rose but invisible to others, either as a secret or physically.