Echo McCool, Outlaw Through Time by Roger K Driscoll
Echo McCool is a magical, medieval girl with attitude. Don't get on the wrong end of her jump kicks, flick-flaks and open-palm strikes. In her own time, Echo escapes death from a poisoned arrow and hides inside a hollow oak tree. She lies dormant for hundreds of years but in the present day she is awakened by Jason Fleeting, a twelve-year-old runaway from a children's home. Together they set out to solve the murder of Jason's mother and to rescue his kidnapped sister Lauren.