Is Pinwheel an optional boss?

I've always killed him, but can you just run past him up the ladder?

No you have to face him. If you look at the spot where the ladder is while facing him you will notice that it is blocked by a wall that moves away after the fight. Plus i like to face him just to gain the kindling ability and gain more flasks.

To be exact, there is a glitch that lets you skip him though, although he is not skippable in the normal way.

Great, thanks.

The Clerics seek kindling, but this only imitates kindling AFAIK.

I have a (very slim) theory of how to skip him, but haven't even bothered trying.
Right before and above Pinwheel's fog gate is the passage you walk through if you want to leave the tomb of giants. And on the other side of the wheel skeleton pit is a black knight with a greataxe.
So, the theory is to lure the black knight towards the passage, and to make him do his heavy attack to throw you into the air, so you land on the ledge of the passage above.
Good luck

^I think there's an easier way!

If I remember correctly, Vageta showed in a video that the game has some sort of fail-safe put in place, where your character is reset, if the game doesn't know where to place you. This happens when you hard-quit the game (from the PSN menu) at a certain moment. In other words: in the same way as the Kiln glitch works, to allow you to skip all the lords, you could skip pinwheel. I think... But this is getting way too much into glitching territory, so I'm not comfortable discussing this here!

Also... I don't see the point of skipping Pinwheel this way, if you can just skip all the lords anyways! There aren't that many things worth getting on the other side of pinwheel that aren't Nito, so unless you're after WoG or the Effigy shield, I don't know why you'd do this. Also, might as well just kill pinwheel! He's a pushover.

Nah, just didn't like the fake Kindling rite; was looking for something else.