Zipper is a friend of Monterey Jack's who the Rangers first met in To the Rescue (Part 2). He is often the 'forgotten Ranger'. But despite this, and in addition to be being by far the smallest in the team, he never acts as such, and one may argue he is as important as any of the other Rangers.
Zipper is a blue insect, and almost certainly some sort of fly, but his exact species is never specified. He wears a red t-shirt which still leaves room for his wings to flap freely, but has no apparel on the lower half of his body. Anatomically, he is more akin to a human than to most insects. His eyes take up most of his head, which in turn makes up half of his frame. This makes his facial expressions easier to tell, and this in turn makes up for his lack of coherent speech.
Due to his aforementioned muteness (unless one is able to make sense of buzzing noises!), Zipper's personality is often left as a mystery. However, this makes his actions all the more noticable in determining the type of fly he is. Zipper has displayed countless times his tremendous bravery and loyalty to the Rangers. As much as he is often pushed into the background, Zipper is always obedient and willing to help, sometimes putting himself in grave danger for the team. Many believe that Zipper's comparitively Herculean strength in fact comes from his enormous heart!
Zipper's role is sadly limited, again partly because of his inability to converse with the others (although Monty - and occasionally Dale - has been known to understand his frantic buzzing).
In certain situations, Zipper has made use of his small size and comparitive strength to help the Rangers out of a tight corner or trap, or to help them gain the upper hand on a foe.
Monterey, it would appear, has a special bond with Zipper. The two have clearly been friends for many years, and Monty feels Zipper is the one member of the team he can trust with absolutely anything.
Accepted Fan Fiction Conventions
Unfortunately, Zipper seems to be as left out in the fanonical universe as he is in the canonical Rangerverse. In many cases, he is excluded altogether from fan fictions. If he becomes involved at all, it invariably involves Monty.