Etrian Odyssey, no spoilers

[T.G. Oskar]

On a separate note:
Are there any benefits to having less than 5 party members?

Nope, not really. You should always have 5 party members in tow. However, there are three quests that require you to have one party member; usually, a Protector or Troubadour can handle that, but it's pretty difficult to finish those quests.