Paola, "со-" co-(con-) "рать" - 1.archaic and poetic for "warfare", "war", "battle". 2. Later meant also "army" (poetically too), so modern Russians mostly know it as a poetic for "army". I don't know which of two meanings was used here. "-н-" - adjectival suffix "-ик" - male unit noun suffix. "-ца" - feminine unit noun suffix. Compared to "-ка" it is... It can be met in words like 'царица' or 'матрица' (matrix). It may have Slavonic origin and is more 'elevated' style than '-ka'. Cf. девка (19th cnetury peasant for girl, now rude for girl), девИца (19th century official for unmarried woman), дЕвица - fairy tale / epic for young woman. So it means "comrade in arms", literally "participant of the same struggle". Moslty used metaphorically for ideological, political, revolutioary... sometimes even scientific 'fights' and struggle. Not necessary violent. Can be used in combination "соратник по борьбе" Sometimes it is just a fancy (jokular or conversely, solemn) word for 'comrade, friend'.

Конкретно в этом предложении - сторонница, последовательница. В принципе "соратник" - союзник, партнер, сторонник, даже друг в каком-то смысле подойдет.

Спасибо вам обеим )

в данном контексте подразумевается "единомышленник", человек таких же взглядов

Соратник-это помощник в каком-либо деле, сторонник, человек, разделяющий твое мнение, товарищ по какой-либо деятельности, единомышленник.