On the island of Kiaku, there were once three original Toa. C4, Ethel, and Semtex. They protected the island and the vulnerable and valuable city, Kiaku City. The city houses hundreds of thousands of Glatorian, Agori, and Matoran. Only a select few could be chosen to be a Toa-Kii. Semtex was chosen to be the third Toa.
Although he is a Toa, he has no real power. Unless super strenght his acceptable. But he has no mask. His power is his muscles, or his robotic muscles that is. Most inhabitants of Kiaku are presumed to be mostly organic, meaning they have only a few bionic parts. However, Semtex has been though a war on another planet entirely and most of him did not make it out intact. He now has bionic limbs. Though, now that e has these bionic limbs, it doesn't mean he wasn't strong before. His limbs just make him stronger
If he wanted to, he could snap anyone in half like a tooth pick, or he could crush them like a tin can. His strength is unlimited. He will stop at nothing to win. He truly is a Toa-Kii.
His silver armor is made of a duradium, gravitanium, and prototanium alloy. It is known to be the strongest metal ever created. His respirator, located on his mask, and his mask is made of the same alloy. The only thing that has been known to break through it was a lava sword, super-heated to the point where it reached the heat of the sun.
Semtex may be strong, but he has one vital weakness. He relies on a respirator connected to tubing that leads to two air tanks that are located under the silver piece of armor on his back. If those tubes were to be cut, or his respirator was damaged he would suffocate and die. The air in his air tanks are pressurized, so even if there was a small cut in the tubing, t would run out in a matter of seconds. He does his best to protect his vital life support, but sometimes it doesn't always work that way...
The way he ended up like that was when he was in the war on another planet. He was captured by the enemy forces and was tortured for information. They made him inhale a poison that damaged his lungs permanently. But he never gave up the information. So they tried to kill him by making him inhale more. He was rescued before he died that day. The doctors fixed him but they told him that he would never be able to breathe the air again, he would only be able to breathe out of an air tank for the rest of his life. that saddened him, but he never gave up.
And not but 500 years later, he became a Toa of Kiaku.