I would like to know if you can use K2 to create a register form and replace the one in Joomla 3.x.

I have no clue about it, it's the first day I am checking K2 lol

K2 extends the one of Joomla! and you can modify it through the K2 override.
It has it's own view file.

Hi Yiota, thanks for the answer. Well... should be one of these buttons in the attachment, right?


thanks

Actually, no. You cannot edit it from the backend. Only through the file itself.
You can create a template override, read here: getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates how K2 template overrides work and modify the file itemform.php

If you want more custom fields than the ones the form already has you can install this K2 plugin getk2.org/extend/extensions/73-user-extended-fields-for-k2 and extend even more the registration form.

Is it possible to add/remove k2 registration fields without installing an additional plugin? How would I go about doing this?

It is not possible without hacking core files and risking to lose all you changes in the next update.
Please use this plugin as a starting point instead getk2.org/extend/extensions/73-user-extended-fields-for-k2