My memory is foggy on that one (it has been way too long) but it's something along the way of calling e.g. MC not with [mc] but with [mc!C]. This will capitalize the first letter of the called variable.
Check the Ren'Py documentation for the correct switches - there's actually a lot more you can do but I have either forgotten or never cared enough to memorize
Edit:
Was curious so went to check.
Quote from
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"The !u flag forces the text to uppercase and the !l flag forces the text to lowercase. The !c flag acts only on the first character, capitalizing it. These flags may be combined, for example using !cl would capitalize the first character, and force the remaining text to lowercase."