THAT'S SO SWEET!!!
MianMian would totally be very close to him on Step 1, as they're this close and Runie seems to understand and like her company, she'll even think of him as her best friend, the one she'll always have a soft spot for, being more gentle with him and actively trying to show him she cares.
For Step 2, MianMian understands him well, she knows how affectionate Runie is, and she reciprocates it easier than in step 1, she'll hug, hold hands, let Runie sleep on her shoulder or lap, she touches very few people, Runie is one of the people she doesn't have this limit, she apreciates him very much and knows she wants him to be in her life forever!
In Step 3, MianMian will always have a spot for him in her heart, like, Runie is one of the most important people on her life, she doesn't let much people see how vulnerable she is, she hates this intimacy, but she doesn't mind If It's him, because she knows she trusts him the most.
Step 4 is like Step 3, but now, MianMian is more open with her feelings, now, she initiates contact too, doesn't wait for him to do so as when she was younger, MianMian now feels completely secure to do so, because, she knows deeply in her heart, she loves Runie just the way he says he loves her too, how could she not love her friends? They're very lovable, especially Runie.