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★ units who can upgrade to 5★ in future updates. The only other current 4★ units who can upgrade to 5★ from the list are Akane and Nikeh; however, their 5★ upgrades are AS (Another Style) versions rather than Other Tales content, and they are made available much later than the former two.
Note that if the player does not take either Miyu or Ciel, they will not be able to access their related Other Tales content. It is still possible to obtain them from Dream Encounters, but this is not guaranteed.
During update 1.4.900, Miyu and Benedict were given away as guaranteed free characters, again making Ciel the ideal choice for the tutorial encounter. As of update 1.5.500, the promotion has expired; Miyu would take priority again as her 5★ upgrade is heavily featured.