Wellfare servants

Generally, no.
For example, every Christmas there's a new welfare servant for the event. The three santas so far are Altria Santa Alter, Jeanne d'Arc Alter Santa Lily, and Altera the San(ta).
They do however reappear during event reruns. The Altria Santa Alter event will have a rerun this year in November.

They will reappear during re-runs but not "sequels". SO Santa alter will appear again during the rerun of the 2017 Christmas event (which should happen just before the 2018 event) but will NOT appear during the actual 2018 Christmas event.