All projects hosted under nikurasu.org are open source and all of their codes are available on Github:
All data saved to make MyMangaDex work is only saved locally and never sent to any server unless an MyMangaDex - Online Save account is created.
An User can delete all of his data at any moment from MyMangaDex.
Users can register if they want, this is not required, and a row identified by an ID and an username they can choose is created.
The password used to login is hashed and is not stored in plain text.
The received and stored data is only what the User wants to save, when reading on MangaDex, and can be stopped by logging out or deleting your account.
The stored data is not shared to any third party and is only stored to help Users when changing devices when using MyMangaDex.
All requests sent to MyMangaDex - Online Save are sent over SSL.