During the tutorial encounter, the following characters will be available to choose from.
The recommended choices are Miyu or Ciel, as they are one of the few 4* who can upgrade to 5* in future updates.
The only other current 4* who can upgrade to 5* from the list is Akane; however, her 5* came at a later update than the other two in the Japanese version, and is restricted to her AS (Another Style) version.
If one was playing on the initial release version, Miyu was given for free, so choosing Ciel was ideal. However, as of version 1.1.100 (21), the free distribution of Miyu has been discontinued. (She is still available along with all the other 4*s listed for the tutorial summon.)