Strike through text After Effects CS6
Text control is very limited in after effects. There is no functionality in the Character Panel to achieve a strikethrough at the moment. 



Using Illustrator

For better text control I'd suggest create your text in illustrator instead:



and import the illustrator file into after effects afterwards:



Note: Editing the text in illustrator is still possible.



Using a shape layer

In order to create a simple stroke above your Text Layer:


Enable the Pen Tool (G) 
With no layer selected, click into the Viewer (this will create the 'first point' of the stroke)
Hold down Shift to make sure that the line, which you are going to create is 'straight'
Click into the viewer again to set the 'end point'


This will create a new Shape Layer. If the layer is selected you can adjust the 'stoke color' and the 'thickness' in the Tool Bar:



Another way is to roll out the Properties of the layer by clicking on the tiny triangle > left beside the layers name, go to Contents > Shape(Number) > Stroke(Number) to adjust the 'color' and the 'width'. 



However a shape layer provides much more control in terms of animation:


Add a Trim Paths property
Create a first keyframe by clicking on the stopwatch of the End value and set it to 0
Move a few frames forward
Create a second keyframe and set End value to 100




Related: https://graphicdesign.stackexchange.com/questions/49643/how-to-use-after-effects-trim-path-with-an-illustrator-path/49975#49975

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