From the Super Mario Wiki
Sumbi is a minor Nimbi that appears in Super Paper Mario. She is one of many Nimbis named for a season or month; in her case, Sumbi is named after summer. She has no important role except for the fact that Bowser frees her from being frozen by using Fire Breath to melt the ice.
That's Sumbi. A female Nimbi. She's been drawn to men who reek of danger lately. Unfortunately, only boring men are drawn to her.