Note: she is not really in Kodomo no Omocha. She is the character of the movie in the show.
Mako was the girlfriend of Hiroto Suzuhara's eldest brother, Masato Suzuhara and friend of Miwa Nakagawa. She, along with Masato and Miwa, went missing for 6 years.
When Hiroto decided to search for his lost brother in the villa that Masato was supposed to have go, and that it was supposedly not found, he met with Mako. Mako lead him to his brother only to find out that both him and Miwa had grew up but not her.