Who is a trickster? Trickster is not a character as such but is a quality of the character. In mythology, Trickster is a god, human hero, or an animal who plays pranks or otherwise disobeys normal rules and norms of behavior.

The shape-shifting who can change shapes, Trickster breaks the rules of the gods or nature, sometimes maliciously, but very often with ultimately positive effects. He is a creator, a joker, a truth teller, a story teller, a transformer.

The Coyote is the Trickster in most Native American cultures.
In African folktales it is Ananse Tortoise or Anansi Spider.
In Europe, he is often the Fox.
In Scandinavian countries, it is Loki.

So Loki is a trickster of Scandinavian folktales - a puzzling and complex character. You either love him or hate him, there is no middle ground. He is involved in numerous pranks and jokes but often, while setting up comical situations, bad things happen and tragedy ensues. For instance, his nasty role in the Death of Baldur, best-loved of all the gods makes it hard to feel fondness for Loki. (see Scandinavian Folktales below)

Loki has a wife - Sigu or Sigurn. Sigurn is a kind woman, far too good and kind for Loki. hey had two sons - Narti and Vali
Loki has two sons more from Angerboda (a giantess) - Giant Fenrir and Monster Jormungand; and one daughter - his chilly daughter Hela.

Loki is the son of male Jotun Farbauti and Laufey or Naal.
He has two brothers - Helblindi and Byleister.

Asgard's Wall
Taken From  http://mythology.wikia.com/wiki/Loki
It is believed that when Asgard's giant mason demanded an unreasonable price for his work - actually he asked the Sun, the Moon and Freya as his wife, it was only Loki who thought about to give him 6 months to build the wall. He thought that the mason will definitely not be able to build it in 6 months, and Loki's plan was perfect. So God agreed, except for Freya who was still part of the bargain.

When a large part of the wall was built long before the 6 months were to be completed, the gods looked at Loki. They threatened him  that he would be killed if the wall will be completed within 6 months.

Loki got afraid. He took a form of a young man and took the horse of the mason away from his master. This made the mason lose the bet. The union of Loki and the mason's horse brought Odin an 8-legged horse Sleipnir. After sometime the wall was built.

Loki Gets Three Children
After the wall was built, Loki became angry with his wife Sigyn, he went to Jotunheimer where he spent some time together with a giantess Angrboda and had three children from her. They were as terrible as their father.

The first one was the wolf Fenrir whose mouth reached from the heavens to the Earth; the second was Jormungandr, the serpent that encircled the Earth; and the third was the goddess Hel (see Hela). After hearing of their birth and the prophecies surrounding them, Odin called them. When they had arrived, he took Fenrir in his custody; he threw Jormungandr into the ocean to grow and gives Hel dominion over Nilfheim.

In the tale of "The Binding of Fenrir", we find that the eldest son of Loki Fenrir and Angrboda were the wolves which ate Tyr's hand.

Loki's Tricks - The Golden Hair of Sif
Why did Loki cut off Sif’s beautiful golden hair? Perhaps it was just a moment of sheer mischievousness but, in any case, Loki didn’t do too badly in the drama that followed.

One day Loki came upon Sif, the wife of Thor, sleeping under an ash tree. Quickly and quietly he snipped off her famously golden hair leaving her head cropped and bare. Naturally, when Thor saw the result, he knew Loki was to blame. Odin All-Father commanded Loki to make amends for such mischief.

So down the Trickster went, through winding passages of earth, to Svartheim, where the dwarves dwelt and asked those master-smiths to make some long golden hair. The dwarves took a gold bar and worked on making delicate threads, bright as sunlight, hammering them over and over again until they were as fine as the hairs on Sif’s head. While he was there, Loki managed to talk the dwarves into handing over two exquisite treasures.

One was a spear called Gungnir, so well balanced and made that it would hit whatever mark it was thrown at no matter how bad the aim of the thrower.. The other was a boat called Skidbladnir that could sail on any sea, but that could be folded up so that it would go into a pocket.

Loki gave the spear to Odin, the ship to Freya and, as for the hair, that shining web held to Sif’s head as if indeed it had roots and was growing there. So everyone was pleased at the outcome of Loki’s prank.

Loki plays his part - A necessary part at the End of the World
You can tell a lot about a god by his names. Take a look at some of the titles Loki is given - Sly One, Lie-Smith, Sly-God, Shape-Changer, Sky Traveler and Sky Walker. This gives you a fair idea of his character.

Loki was responsible for the death of Baldur the Beautiful, the most loved of the gods among the Northern Halls. This was a terrible tragedy for all of the gods and in particular for his mother, Freya, who went to such trouble to keep her son safe.

However, Baldur is one of those who have to be reborn at the End of the World and, of a necessity, someone has to be dead before they are reborn. So Loki makes it possible for Baldur to be reborn to fight at Ragnarok, the Twilight of the Gods, when the world will come to an end.

Loki Attempts to Hide
After killing Baldur, Loki knew that he had now done what could never be forgiven. He turned his back upon Asgard, and wandered far into the thick woods, and covered himself with the deep waters; he climbed to the tops of misty hills, and crouched in the dark of hollow caves; but above the wood, and through the water, and down into the darkness, a single ray of calm, clear light seemed always to follow him, and he knew that it came from the eye of Odin.

Then he tried to escape the watchful eye by disguising himself under various shapes. Sometimes he was an eagle on a lonely mountain-crag; sometimes he hid himself as one among a troop of timid reindeer; sometimes he lay in the nest of a wood-pigeon; sometimes he swam, a bright-spotted fish, in the sea; but, wherever he was, among living creatures, or alone with dead nature, everything seemed to know him.

Loki built himself a house near a narrow, glittering river which dashed down from a high rock into the sea below. He took care that his house should have four doors in it, that he might look out on every side and catch the first glimpse of the gods when they came, as he knew they would come, to take him away. Here his wife, Sigurn, and his two sons came to live with him.

The Gods Catch Loki
One day Loki spied in the distance the whole company of the gods approaching his house. The sight of them pierced Loki with a pang that was worse than death.

He rose without daring to look again, threw his net on a fire that burned on the floor, and, rushing to the side of the little river, he turned himself into a salmon, swam down to the deepest, stillest pool at the bottom, and hid himself between two stones.

The gods entered the house, and looked all round in vain for Loki, but they saw the remains of the fishing-net in the fire. Odin knew at once that there was a river near, and that it was there where Loki had hidden himself.

Loki, seeing the danger he was in, swam and leaped as quick as a flash of lightning, but not quickly enough. Thor stretched out his hand and caught him while he was turning in the air and, although Loki wriggled and slithered, the Thunderer grasped him tightly by the tail and, holding him in his hand, waded to the shore.

There Odin All Father and the other gods met him and, at Odin’s first searching look, Loki was obliged to drop his disguise. He assumed his proper shape before the assembled lords and one by one they turned their faces from him for, in looking at him, they seemed to see over again the death of Baldur the Beloved.

Loki's Punishment
There were high rocks looking over the sea near Loki’s house and one of these, higher than the rest, had four projecting stones, and to these Loki was bound. His two sons had by this time been transformed into wolves, had fought with and destroyed each other.

"With their sinews we must make a chain to bind their father, and from that he can never escape,” growled Odin. It was done. A rope was made of the dead wolves’ sinews and, as soon as it touched Loki, it turned into bands of iron and bound him immovably to the rock. Secured in this manner the gods left him.

Loki was bound with three bonds made from the entrails of his son through holes in three upright slabs of rock, the first under his shoulders, the second under his loins and the third under the backs of his knees.

But his punishment did not end here. A snake, whose fangs dropped poison, glided to the top of the rock and leaned his head over to peer at Loki. Every few moments a burning drop from his tongue fell down on Loki’s face.

In all the world there was only one who pitied him. His kind wife Sigurn ever afterwards stood beside him and held a cup over his head to catch the poison. When the cup was full, she was obliged to turn away to empty it, and the deadly drops fell again. Loki shuddered and shrank from them, and the whole earth trembled. So will he lie bound till Ragnarok.