This is possible, but only by adding this functionality with a Slack app.
Standard Slack functionality
The standard Slack does not support different aliases for a user. So if you want to change your persona on Slack you easiest approach would indeed be to create multiple user accounts.
To make it easier to use multiple user accounts on Slack I would recommend to run each user in a different browser. (e.g. User A in Chrome, User B in Firefox, User C in IE). Since Slack is storing the user authentication with cookies, you can run all users simultaneously that way. I do this all the time, e.g. to test workflows between multiple users on one machine.
Using a Slack app
But you can use a Slack app for posting your messages. That app can include an individual username and icon in the message, which would allow you to post as an NPC.
This works with the web API method
chat.postMessage by providing a custom
icon_url property. So you could make a simple Slack app that can be called with a slash command (e.g.
/npc mario "Hi this is Mario, have you seen the princess?" or use a Dialog for that) to post in the current channel as an NPC.