I want to say either Luna or Sirius. To be honest Tonks was more rebellious then chill, Fred and George were way to mischievous to be chill. Harry was always to crazy things and stressed about everything, Dumbledore was kind of chill but he had kind of a crazy side to him to. So ya I'd say Luna or Sirius because even while fighting they were always calm. Well except that one time in Askarban.......