If you are asking how to add a menu item to the left side-bar that opens a passage that displays the characters stats then you can do that by adding the StoryMenu special passage to your project and place a link to your Stats passage in it.
[[Stats]]
You have two options when it comes to exiitng your Stats passage, you can either use the Undo navigator button found at the top of the left sode-bar or you can include a "Return" link in your Stats passage.
If you choose to add a "Return" link then you will likely want that link to always return the reader to the last non-menu related passage they were viewing. To do this you will need to do the following.
1. Edit your Stats passage and assign it a menu Passage Tag, then add the following link to the content of the passage.
<<link "Return" $return>><</link>>
2. Add the following code to your project's Story Javascript area, it updates a $returns story variable if the current passage being shown doesn't include a menu passage tag.
predisplay['set-return'] = function (taskName) {
if (!tags().includes('menu')) {
State.variables.return = State.passage;
}
};
3. If you add other menu related passage to your project then you will need to also assign them a menu passage tag.