If "Use Onedrive" is checked in Kakitai's settings it will request access to the Onedrive associated with your windows account or if no account is associated with windows it will request you to login. The authentication is handled by Microsoft's Onedrive Api so credentials are not stored by Kakitai. This however gives Kakitai acesss to your Onedrive, you can revoke access at anytime by logging into your Microsoft Account and removing Kakitai from your consent list.
Kakitai will then store copies of your Progress Data File (Data.xml) and your Custom Data File (CustomData.xml) to your Onedrive. There will never be any personal data or data that uniquely identifies you stored in these files only the data necessary for sharing your progress between multiple devices and will not be accessable to anyone other than you. The data stored by kakitai can be viewed in your onedrive in a folder called "Apps/Kakitai" and Kakitai does not accesss any other files on your Onedrive other than the files it creates.
By default when you create a support email from within Kakitai it will include some basic information about your device and a copy of your Settings, Progress Data and Custom Data files. These files contain no personal information or information that uniquely identifies you and will be used only by the Kakitai author to help improve Kakitai. This information can be edited or removed from within Kakitai before the email is sent.
Kakitai forums requires registration to submit posts, email address is requested but only used for account recovery and notifications. Data requested on registration is not shared with any 3rd parties and only used for usage of the website. Other optional information submitted to forums is available to the public so posting private information or information that can uniquely identify you is not recommended.