Can I use font characters as markers?

You can’t modify the markers that are built into KaleidaGraph. One thing you can do is add text error bars to your plot. Text error bars are normally used to annotate the points in the plot. You could create a Line plot, hide the original markers, and add text error bars to the plot.

As an example, you might create a text column with the letter a in each row. Once you add this column as text error bars, you can double-click the text and change the font to Zapf Dingbats or some other font that has different symbols in it.

The manual and help file contain information on adding text error bars to a plot. To use them as markers, you would want to add them as X error bars. You would also want to make them single-sided and have the text centered (using the Center Text option).