As you say, there are 2 NPC's already available in the Adventurers Inn at the start of the game: Devlin Arcanus and Elsbeth Lamentia. These are the only ones available when you start a new game - there is no third one. There are 2 further NPC's available to find in the first map area: Fredrick Talimere the Cleric, & a Knight (forget name). Talimere is found in the village where you start (Blood Drop) but he won't join until you complete a small task. Talking to him will trigger it. One thing to watch out for when talking to NPC's in MM8 are chat options that change to other options once you ask about them. So asking about "Cataclysm" might change to "Quest" once you click on it first time. This is the case with Talimere: you may think you have already clicked on all options, but try again (and watch for ones that change).
The Knight is available immediately once you find him (although you need to complete Talimere's task before you'll be able to find him).
If you tell us which NPC you are referring to, we can help further