Nurse Ramirez is a nurse with nice nails. She reassures Mia's grandparents and helps Adam find Mia. Mia likes her because she doesn't follow the rules and because she allows Adam to visit her when the rules don't allow it.
A nameless social worker is one of the few people Mia doesn't like. The social worker warns Mia's grandparents about their granddaughter's post-accident condition, and Mia sees her as talking to her grandparents like they are "children" (8.20). However, when the social worker isn't being extra sensitive to them, Mia gets mad at the social worker for that, too.
Ghosts are moody.
Brooke Vega is a "Punk-rock diva" (1.22) who creates a distraction to help Adam see Mia. Brooke is punk rock because she offers to take her top off in public. Simon is an English "queer" (11.88)—ouch—who is Mia's friend and cello rival at music camp. We only see him in only one chapter, where he's there pretty much so that we can see how much better Mia is at cello than anyone else.
Finally, Willow and Henry are family friends. Mia likes Willow because she breaks the rules. And in flashbacks, Henry follows the same trajectory as Dad, giving up his hard-rock lifestyle to raise a child.