If Ryu is hit far enough offstage where he needs to side b to recover, chase him and be prepared to hit him out with backair or downair. This situation is generally death for Ryu. If he is recovering low, a spaced down air will intercept his up b. Up b intangibility ends at frame 6, so anywhere in the middle or end of the up b will be vulnerable. His only aggressive mixup is rising side b (which consumes his jump). Otherwise, Rosalina is not at much risk while looking for an edgeguard.
Rosalina & Luma