Some tips for Ralph & New Kitten "Bella"

Well im seeking your advice about Ralph and our new kitten, "Bella". She is fearless and gets on with Florrie really well-they will often cuddle up together, snooze, lick each other etc-perflick! But with Ralph it's a different kettle of fish..Bella has the roaming of the house, as she should, and ventures downstairs beyond the stairgate daily..she loves downstairs, sometimes Ralph can be fine..others he can be a right demon, n won't stop trying to get at her..usualy when she's walking around on the floor. He jumps on her..is far too rough and far to big to do so, its starting to worry me that one day he might seriously, yet unintentionaly, hurt her.
Any advice?
It seems to me to be more of an issue when Bella is walking around etc, than when she;s sat on the sofa or whatever, but i can't take the risk of Ralph doing any serious damage.
So any advice?