As for customizing, there is no add-on for that sort of thing. It's built-in. You create the graphics file (check the RTP folder and the Charset folder to get the spacing correctly) and then simply "import" them into your game using the "RAW material editor" under the tools menu of the program.

The graphic file you make should be either BMP or PNG. MS-Paint is sufficient for that sort of thing.

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For that story, here are the things you would need.

Make a character event.

Page 1:
(Same level as hero, stand still, push key) : We'll call this one Emily, but whatever you call it will be fine.
Page 2:
(Keep the character animation blank, and under the event conditions, you check one of the Switch boxes, and select a switch you want (make sure to name it something like "FirstMission". Also set to Under Hero.)


Emily would need these events on Page 1: (Though the Face events might be left off if you want)

Select Face: (Whatever face you want for Emily)
Show Message: "Emily> Hey BlueCube, there's a monster attacking the village directly to the south!"
Select Face: (BlueCube's face)
Show Message: "BlueCube> Well, alright, but there's this forest blocking the way.."
Select Face: (Emily's face)
Show Message: "Emily> Don't be a wuss! I'll show you the way!"
Change Party: Add Emily
Set switch: FirstMission

At that point, the Emily event should be gone in the game. Hope I didn't overlook anything...