Miki is the team's lifeline; if she goes down, expect everything to fall apart. That said, you have to protect her at all costs. Her wide variety of healing spells are potent enough but if further enhanced with equipment and Accessories that increase her healing potency and casting speed, she can pretty much chain healing, recovery, and revival spells fast enough to get your team out of the bind.
Being a healer doesn't constrain her from having very effective and powerful offensive spells as well. Her fire and light-based attacks can dish out huge damage when she's not busy healing.
For the spreadsheet Skill List, follow this link.