Help Desk Ticket Reference: https://c.na7.visual.force.com/apex/Case_View?id=500A0000004y9fG&sfdc.override=1
Steps to Reproduce: Create a declarative model like:

declare Foobar
	testChar: char
end

Now just try to create any rule using the Guided Editor, where you try to set a value for the field testChar.
Workaround: Workaround Exists
Workaround Description: Use free form DRL
securitylevel_name: Public

If a fact has a char type for a field, is not possible use the Guided Editor to insert a value for it. Curiously, it does not support entering alphabetic characters, but allows numbers and special characters like "[}(*".