Change Font of a Specific Character through Text Animator

Text animators cannot change fonts. Expressions can only change the font of the entire text layer. Expressions can count the number of characters, and Text Animators can be set to use the number of characters in a layer.

A simple expression based on the number of characters tied to the start and end of one layer and the end of another will show all but the last letter on the first layer and only the last letter on the second. The only downside to this approach is that you will have to set both layers to be Right Justified. If you need to make the pair of text layers look like they are Left Justified, then you will have to add another expression that modifies the layers' position based on the text layer's width.

Throw in some parenting, and you should have the solution. Depending on the font, you may need to adjust the horizontal position of the second layer (the last character) to fix kerning issues that may come up when you change fonts. The comp would look like this:

Here are the expressions:

//Top Text Layer/Animator 1/Range Selector/Start
ref = text.sourceText;
num = ref.length;
lChr = num -1;

//Top Text Layer/Animagot 1/Range Selector/End
ref = text.sourceText;
num = ref.length;

//Top Text Layer/Transform/Position
w = sourceRectAtTime().width;
[value[0] + w , value[1]]

//Second Text Layer/Text/Source Text
thisComp.layer("Main Text").text.sourceText

//Second Text Layer/Animator 1/Range Selector/End
ref = thisComp.layer(index - 1).text.sourceText;
num = ref.length;
lChr = Math.max(num)-1;

The Text/Animator 1/Opacity for both layers must be set to zero, and the Animator 1/Units must be set to Index.

That should get you started. I've included a project file you can fiddle with.