We don't have synonyms for remainer yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are remain, remainder and reminder.
(n) :
(UK politics) Someone who supports the United Kingdom remaining within the European Union.
(n) :
(UK politics) Alternative letter-case form of Remainer. [(UK politics) Someone who supports the United Kingdom remaining within the European Union.]
(n) :
Obsolete form of remainder. [A part or parts remaining after some has/have been removed or already occurred.]
(n) :
(UK politics) Someone who supports the United Kingdom remaining within the European Union.
(n) :
(UK politics) Alternative letter-case form of Remainer. [(UK politics) Someone who supports the United Kingdom remaining within the European Union.]
(n) :
Obsolete form of remainder. [A part or parts remaining after some has/have been removed or already occurred.]