I came across the word der Mitstreiter in an article in Hinz&Kunzt about Marily Stroux from Wohnschiffprojekt Altona e.V. winning a prize for her work with refugees. It was the occasion for one of my incorrect initial mis-syllabifications. Der Mitstreiter is mit + Streiter (that is “with” + “contender” or “militant” or “champion”) not *mits + Reiter, although the idea of someone who’s “riding with” someone else does capture the meaning of Mitstreiter –”comrade-in-arms” or “ally” – in a poetic sort of way.