Berserk.

Puck is there mainly for comic relief. Since the manga has a very dark plot, the artist probably wanted to include Puck as a balance to dish out some humour along the way.

I kinda like Puck's antics actually, especially when

Spoiler:

he laid on top of a flying crow and slowly plucked off the fur on the poor bird's head (all the while thinking up funny sketches of the 5 God's Hand) until it was bald. Then the crow angrily pecked Puck's head until he bleed, lol.

Jam it back in, in the dark.

Originally Posted by Gechmir

Puck has his importance, ie: healing. As of late in the manga, though, he has strictly been the comic relief.

True, plus there are new characters now to fool around with him. I think Puck is important in that he heals both the physical and mental state of Gatts. Remember in the early part of the manga Gatts was close to being pyscho from all the hunting and killing of the monsters? Then after he met Puck, his mood livened up and he began to think straight too...otherwise he will really become berserk. However as of current now, this responsibility has been taken up by another character, which is

Spoiler:

the young witch and probably that unknown ghost child: when gatts fully puts on his armour to become the berserk warrior, these two have the ability to pull gatts back into reality.

How ya doing, buddy?