Marina is another child from Yomogi Nursery School.
In episode 6 all the class leave on a trip to the park, Kobato ends up playing at hide & seek with kids but since she's worried about teacher Sayaka's problems, Marina notice this and thinks that Kobato is sad. So during hide & seek she "disappear" and let anyone getting worried on searching her, when she was just searching a four-leaf clover to give Kobato for make a wish, but she ended up dreaming under a tree.
Kobato finds her and cheers her up by finding the four-leaf clover into her hair.