That one background pony from one episode that did literally NOTHING for the plot. You know the one!! No? Seriously though, this is a really tough decision. I'll take this as "a main or secondary character that had at least a semi-significant role in one or more episodes" and say the head of the Yaks. Can't say their episode is one of my favorites so yeah.
The Younf 6 felt forced in just like this whole school stuff and they had little personality and few likeable traits.
