Sesshoumaru (pronounced ses-SHO-maroo)
Sesshoumaru is Inuyasha’s half-brother, by the same father. His mother (who no one seems to know anything about) was a youkai, making him a full youkai as well. After his father was killed, he was made Lord of the Western Lands, and he keep s the other demons off his territory.
You can always tell him on the show because of several points. He wears much nicer clothes that anyone else. He has armor across his chest, with a spiky shoulder... Thing on his left shoulder. On his right shoulder lays a white thing known only as 'the fluffy.' (more on that in a second) His hair is long and white, and often seems to blow out behind him of it's own accord. On his face are four reddish stripes across his cheeks, and a blue moon on his forehead. His features are a bit feminine, and almost always have the same expression of casual indifference. He is always followed around by a little toad-demon named Jaken. Oh, and he is missing his left arm.
Sometimes his eyes will glow a bright red, but other than that, he has no easily recognizable transformations. He can turn into a giant white dog, but in that form you can only tell that it is him because of the blue moon.
Sesshoumaru hates his half-brother with a passion. When he was the rough equivalent of 15, his father fell in love with a human, who Sesshoumaru thought of as inferior. He left on his own, no one knows where he went. He sees his father as abandoning him for a lesser race, and his brother is the incarnate of that abandonment. What was worse, when he went to search for his father's tomb to retrieve from it the tetsusaiga, he found that his father had left his tomb in a black pearl, residing in Inuyasha's right eye. Even once he got the pearl and entered the tomb, he was unable to touch, let alone draw, the sword from it's resting place. When Inuyasha and Kagome followed him there, he found that Inuyasha was able to touch the sword, and Kagome able to draw it. This did not bode well. The situation worsened after he fought Inuyasha, and instead of easily defeating the 'worthless hanyou,' he lost, losing his left arm in the battle.
This has made him rather uncertain of himself, if he was unable to kill either a human or a hanyou. He isn't sure as to what his father was up to, he was certain the sword was meant for him, but he was unable to even touch it. He can't understand why killing a worthless girl made his brother angry enough to be able to fight dozens of times better.
He has made his way through life showing very little emotion, having, possibly, very little emotion. He seems to have little to do other than try to defeat his brother, and with each defeat he becomes less and less stable. Rin has helped this a bit, more on that as I learn it.
Attacks: Sesshoumaru has no named attacks as of yet, but he does have several useful weapons.
The first is a yellowish energy-whip which he grows from his fingers and can use from almost 30 feet away. His fingernails are poisonous, and in he can make his hand radiate the poison. In his dog form, his drool can vent poisonous gases, which even Inuyasha can't stand for long.
He can wield the tetsusaiga much better than Inuyasha, and is always trying to steal the sword from his half-breed brother.
The Tenseiga is not technically considered an attack, seeing as it is unable to do harm, but there is no where else to put it, so this is where it's going. The sword tenseiga was left to Sesshoumaru by his father, for which he is ungrateful. The sword can heal, but not hurt. It has the ability to bring people back from the dead, as he did with Rin. Even when Sesshoumaru cut Jaken completely in half with the sword, Jaken was still fine. When Sesshoumaru was hit with the full brunt of tetsusaiga's attack, he survived with no noticeable scars or side effects, because of tenseiga's protection.
The other sword is the Tokijen, a sword made from the fangs of the demon who broke the tetsusaiga. Sesshoumaru figured that these fangs would create a sword even more powerful then the tetsusaiga. Plus, it is not manly to go around with just a sword that helps people.
And now everybody's favorite weapon: Mokomoko-sama. Affectionately dubbed 'fluffy' by the fan community, the large white thing carried over his right shoulder remains an enigma even to Romiko. Some people believe it to be a tail, but this is not the case, because at points in the manga, both sides are drawn and completely visible. The other theory is that it is some kind of pillow or blanket or something, but this also can't be true, because early on in the series, Sesshoumaru uses it to grab Inuyasha and throw him against a wall. Also, it is progressively getting larger throughout the series. There is a theory that it is a parasite and he will eventually e eaten, however, I don't think that this will hold up for long.
Weaknesses: Again, Rin, maybe. He has no arm, that can be a bother at times, although he can graft others to him. He is prone to shock, as well, and usually expects things to turn out as he plans, although this is often not the case.

Jaken (JAH-ken)
Jaken is the lackey of Sesshoumaru, and they are quite a pair. Imagine the opposite of Sesshoumaru, and that is Jaken. He's short, green, and has a beak. He wears brown clothes, with a tall brown hat, and carries the staff of two heads.
Jaken was once the ruler of a clan of demons like him. They were in a fight with a bunch of much bigger, more powerful demons, and they were losing. Jaken was about to get killed by the ruler of the other demons, when Sesshoumaru showed up. The battle field was directly in his path, but hell if he was gonna walk around it. He informed them calmly that they were in his way, and when they didn't move, struck the leader dead, freeing Jaken. He then walked on. Jaken left everything behind to follow him.
When Sesshoumaru realized that Jaken was following him, he gave him the staff of two heads, and told Jaken that if he could use it, he would be allowed to follow. Obviously, he learned how to use it.
Jaken feels that he owes Sesshoumaru his life. Sesshoumaru could care less, but does allow him to stay around. Jaken has also taken a liking to Rin, although he tries to hide it. It's his job to make sure Rin doesn't get in trouble, to lean Aun around, to think up devious plans, and to basically be am evil little minion. It was his idea to use the un-mother, which should have worked.
He also gets beat up alot, what with Sesshoumaru knocking him over the head all the time, but if he minds, he doesn't show it and continues to follow his ungrateful master to whatever fate may come.
Attacks: Jaken's main attack is the staff of two heads, which allows him to shoot fire at his enemies. He's also rather clever.
Weaknesses: Jaken is tiny. Without the staff, he's dead meat, and even Miroku could get a few knocks on his head.

Rin
(If you can't figure out how to pronounce this get the hell off my site)
Rin is a little orphan girl. She wears a yellow and orange checkered dress. Her black hair is up in one pigtail and down on the other side. In the village where she lived, she lived by stealing fish, and got beat up by the village men. (Probably the women and kids, too) When she was out in the woods one day, she came across Sesshoumaru, who was hurt pretty bad by the kaze no kizu. She brought him food and water, which he didn’t take because he doesn’t eat human food, but it’s the thought that counts, especially since he could have killed her in a second.
When Koga attacked her village, she was killed by wolves. A little while later, Jaken and Sesshoumaru found her, and Sesshoumaru brought her back to life with the tenseiga. Ever since, she’s been following him around, mostly the same way Jaken does. She gets in trouble a lot.