According to dict.leo.org:
- Das Mittel, sg. (means; aid) has the plural form die Mittel.
- Die Mittel, pl. (capital; funds) lacks a singular form.
Which kind of Mittel is die Redemittel? Does it exist in singular?
Redemittel (also: sprachliche Mittel) is both singular and plural. It derives from Mittel as in means.