The "extra" room below the baseline is set in the font for descenders. If you really, really don't want it there, convert the letter to outline text. You will no longer be able to edit the character, but there ya go.
Take care, Mike
With type (not characters) selected, go to Effect > Path > Outline Stroke.
In Preferences > General or in Align Panel flyout menu, activate Use Preview Bounds.
Your letter should align as you want.
Yeah, I get the need for this question though because sometimes/often you don't have descenders and it ruins the alignment in those cases. I too wish there was an easy way to redefine this behavior on the fly.