The FeministWiki doesn't save anything about you other than your username, your password, and who added you as a member.
Your username needn't correspond to your real name. If it does, note that other members can see it in some places such as the list of names in the chat service: https://chat.feministwiki.org/ Non-members cannot see a list of members, but if you edit the wiki, post on the forum, etc., then your FeministWiki username will be publicly visible. If this is a problem for you, please contact the technician to change your username to one that doesn't relate to your real identity.
Your password is not saved in plain text. Rather, a "salted SHA1 hash" of your password is saved. This means that even in case of a data leak, attackers won't immediately know your password, though it's technically possible for them to figure it out if they try very hard. (This applies to most websites; the FeministWiki is not any less secure in this regard than other websites that require you to log in with a password.) All FeministWiki services use encrypted communication, so your password doesn't travel over the network in plain text either.
Who added you as a member is only visible to the technician, if she/he cares to look it up from the internally kept database.