Reselect the 'a' character, and now choose Format > Font > Baseline > Subscript. You want to return this to normal, so click the ¶ style selector on the toolbar again, and choose default. Leave the two checkboxes unselected, and click Add. A sheet will drop down allowing you to name this style, and I suggest you call it Superscript.

Leave Document Styles selected, and simply click the Add to Favorites button. In the styles panel, you will see the superscripted letter 'a' and below it, the current font info and the term Raised. On the left end of the TextEdit toolbar, there is a ¶ selector button.

Select the letter 'a', and then choose Format > Font > Baseline > Superscript. Type the arbitrary text 'ka' (without the quotes). Mail does not offer any Superscript, Subscript, or Strike-through style capability, but you can first create them in TextEdit, and then these styles will be available in Mail.