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The title character. Inuyasha is hotheaded and stubborn, but he has his good points, too. Born to a youkai father and human mother during Japan's Sengoku era, he was sealed by Kikyou for fifty years before being released by Kagome and becoming her travelling companion. He's also fond of junk food.
Because he is a hanyou, Inuyasha temporarily loses his youkai powers and becomes a normal human from time to time, as shown to the right. Inuyasha's time is the night of the new moon. While human, he can't use Tessaiga's powers and is largely powerless himself. However, he's fairly strong and tough for a human, and as bad at giving up as ever, so he gets by, especially since he now has help.
Although Inuyasha initially seeks to take the Shikon no Tama and use it to become a full youkai, this becomes less important to him as time goes on. More important things come up, like stopping Naraku, but there's more to it than that. Part of the reason seems to be that Kagome likes him the way he is. The clearest change in his attitude, however, comes after he starts having incidents in which his youkai blood takes over for a time, turning him into a berserk bloodthirsy killer, shown at left. The more this happens, the less he likes it. As he puts it in episode 52 (chapter ??), "The youkai I wanted to become... the power I wanted... isn't like this!" While Inuyasha's long-term goals are unclear, becoming a full youkai doesn't seem to be among them any more.
Tessaiga is very picky about who uses it and how. As mentioned above, full youkai cannot even hold it. However, the sword normally appears to be nothing more than an ordinary old, dull, scratched, and dented katana, as pictured to the right. Inuyasha accidentally activates its true power during the battle with Sesshoumaru. After telling a worried Kagome, おれがおまえを守るっつってんだ (I'm saying I'll protect you), the sword begins to pulse, and Inuyasha gets a feeling from it completely different from before. On his next swing, the sword activates, transforming into a massive sword in the form of a fang (ep.7/ch.18). Only a hanyou like Inuyasha has both the ability to wield the sword and the power to activate it, and even then the sword refuses to activate unless the holder uses it with the will to protect humans.
Though strong as swords go, Tessaiga doesn't seem to live up to its reputation of being able to slay 100 youkai in a single swing. That is, until Sesshoumaru, returning in a later battle with a human arm grafted on, seizes the sword and demonstrates an attack that sends gashes of energy into an oncoming swarm of lesser youkai, blowing them all to pieces (ep.18/ch.??). With Kagome's help, Inuyasha manages to get his sword back and force Sesshoumaru to retreat, but can't figure out the attack for himself. Not until a later encounter, when Inuyasha is blinded by Sesshoumaru's poison and has to rely on his nose, does he smell the clashing swirls of youki and realize that he has to swing Tessaiga through the spot where they meet to unleash 風の傷 (かぜのきず), shown to the left, which quickly becomes his favored attack (ep.35/ch.??).
Toutousai manages to repair the sword with Inuyasha's fang, but the reforging means that Inuyasha can no longer rely on his father's strength. Tessaiga becomes too heavy for Inuyasha to wield effectively (ep.44/ch.??). After being stuck with this for a while, going berserk a few more times, and realizing the gravity (no pun intended) of the situation, Inuyasha takes Toutousai's advice and challenges a dragon that his father was only able to seal, not destroy. The idea is that if he can beat an opponent his father couldn't, he will have matched his strength, and Tessaiga will again be light enough to use. During the battle, Inuyasha is disarmed and goes berserk again, but struggles against his youkai blood to retake the sword, which, as if in response to his victory over his own nature, pulses in response and grows lighter. As the dragon continues to fire youki blasts at him, Inuyasha instinctively strikes at a suspicious point in the blast with Kaze no Kizu, unintentionally unleashing Tessaiga's 爆流破 (ばくりゅうは), shown to the right, and returning the attack at amplified power, and carving up the dragon with relative ease. Not only is Tessaiga finally light enough to use normally again, but it also gains the ability to use Kaze no Kizu at will, without having to worry about striking clashing youki fields at the right point (ep.54/ch.??).
Not long after, Naraku begins defending himself with a protective barrier. To combat this new ability, Inuyasha seeks a way to break through barriers. Eventually, Toutousai explains that when Tessaiga slays a powerful youkai, it gains additional powers from that youkai, which is why Tessaiga became stronger after felling the dragon. After some searching and complications, Inuyasha is able to infuse Tessaiga with the powers of a youkai that can erect powerful barriers, and Tessaiga becomes able to transform into the 結界破る赤い鉄砕牙 (けっかいやぶるあかいてっさいが), also called 結界破りの赤い鉄砕牙 (けっかいやぶりのあかいてっさいが), barrier-breaking Red Tessaiga (ep.74/ch.??), shown at left.
Unfortunately, after a while, Naraku strengthens himself again, and manages to come up with a barrier that even the Red Tessaiga can't pierce. In the search for the final Shikon shard, Inuyasha and the others come across Housenki (kanji?), who, after testing Inuyasha, deems him worthy of inheriting his powers, and Tessaiga becomes able to use 金剛槍破 (こんごうそうは), shown at right, which combines a Kaze no Kizu-like attack with a barrage of diamond shrapnel. Naraku's barrier can't stand up to it, though it can slow it down enough to keep the attack from obliterating whatever's behind it. However, it quickly becomes obvious that merely ripping Naraku to shreds isn't going to kill him... (ep.157/ch.325)
Seeking a way to make Tessaiga yet stronger, Inuyasha winds up in a battle against a youki-absorbing sword. In the course of the battle, most of Tessaiga's youki is sucked up, and it's nearly destroyed, but Inuyasha manages to come out on top through sheer force of will. Tessaiga regains its full strength, and aquires the new transformation 竜鱗の鉄砕牙 (りゅうりんのてっさいが), Dragon-Scaled Tessaiga, shown to the left, which can directly absorb an enemy's youki, debilitating the opponent and negating youki-based attacks (ch.391). However, this new ability is, at least for now, more than Inuyasha can handle (ch.399). It remains to be seen whether or not he will be able to master it...