Interesting filler/side characters

Many people seem to hate filler episodes, being rather pointless in the way that they don't contribute to a main plot. Characters in episodes like these are ones we will probably never see again either, and therefore there may be no emotional attachment to them, or reason to even bother with them.
However, some of these characters are pretty interesting, despite having as much use as a lump of dirt.

- Kakero/Kagero (sp?) from a Naruto episode was one of those characters. I think it was sometime after the 100 episode mark where this multi-part person caught my interest:

Spoiler:

First, we see Kakero as some kind of pugly person that had a gigantic ground vortex attack which just about decimated the nearby house. He disappears for a while after the one ninja girl tends to Naruto.
After that, surprise! Kakero was actually a girl with chakra wings who actually looks anything but pugly. She then uses another devastating attack called "Ephemeral Wings", despite the fact that she dies sometime after using it.

It might be hard since most of the time, filler characters aren't very memorable at all, but do you have any that stick out in your mind?

Jam it back in, in the dark.

Well, to tell the truth, the "special" episode of Elfen Lied, which is more of a late filler for me, was really interesting in the further aclaration of

Spoiler:

why Lucy behaves the way she did with humans, besides being trapped, tortured, among many other things by Kurama, and showing that it's not only Kouta's "fault" for what happened in episode 10, if I recall correctly, making Lucy kill Kouta's cousing if I'm not mistaken.

Can't recall anything else than this.

There's nowhere I can't reach.

Hm... maybe he wasn't exactly a filler character, but Haku from Naruto was pretty darn interesting. Just when I thought he would be one of the main characters, they

Spoiler:

killed him off

yea...

How ya doing, buddy?

What the hell? Haku wasn't a filler character at all. Dumbest reply ever right there.

As for interesting filler-characters, pretty much everyone in the G-8 filler-arc in One Piece was pretty cool for a filler.

I am a dolphin, do you want me on your body?

The thread title says "filler/side" characters. Haku wasn't a main character or filler character, so he fits in the side character category.

I was speaking idiomatically.

I still remember the Tucker father and daughter characters from Fullmetal Alchemist. Their story surprised me very early in the anime, and it was interesting to see how the truth within that family was revealed.

What kind of toxic man-thing is happening now?

As I'm slowly beginning to realize that I want to watch Twelve Kingdoms again, I remember liking King En and Enki, the two arguably strongest supports of Youko Nakajima, the main character. King En was pretty charismatic and a really great leader, but it's nice to see a guy like him partnered with a rather rebellious and upstart Enki.

FELIPE NO

Fudo (was that his name?... the man with a metallic claw that can predict anyone's future actions by some minutes) from Get Backers, I really found him to be a pretty interesting character but unfortunately the series never gives you any detailed information as of what he exactly is and the reason he is so obsessed with killing Nido... however his

Spoiler:

death

was a very well done one in my opinion.

And the whole Aristocrats deal from Wolf's Rain (Darcia, Yagara, Okam, etc.) sounded very interesting, but they never got into much detail to it either, at least I would have liked to know more about the back story of the Darcia family.

What, you don't want my bikini-clad body?