Shirly is a middle aged nun who has spent her entire adult life serving the High Priest at Indevs Church. High Priest Perry has put a lot of trust in her, which shows when he asks her to join Mark and Tom on their journey to investigate the elemental coins.
There are two ways in Shirly can support your cause. First of all, talking to her in your castle will always heal both the HP and MP of your allies completely, as well as curing any status ailments.
Secondly, when you take her along, she will always restore a little bit of HP after every battle. By doing so, she will relieve a lot of stress off of your healer.