Keep fixed symbol on a source text linked expression
To add text items together in Javascript is super easy, you just use the + operator, which is 'overloaded' so that when the either term in an expression x + y is a string, the output is the first and second terms concatenated, as a string.

So you can just use + "%" to add a "%" symbol to the end of a number. Works for any character, e.g. 123 + "-foo" would result in 123-foo.

TL;DR:

effect("Slider Control")("Slider") + "%" 


Note that the result of the slider will be a float (i.e. a number with lots of decimal places). That might not be what you're looking for. You can round it to an integer by using the Math functions, specifically Math.round (rounds to nearest integer) or Math.floor (always rounds down).

Math.round(effect("Slider Control")("Slider")) + "%" 


For extra credit: If you did want some decimal places, say 2 decimal places, then multiply the number by 100, round it and then divide by 100:

Math.round(100 * effect("Slider Control")("Slider"))/100 + "%"