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Xelloss only appears in Slayers NEXT and Slayers TRY, and his missions and objectives are different in each series, although his cause is always for evil. Xelloss was created by Beastmaster Zelas Matallium and therefore he obeys only her. Xelloss is not allowed to use his powers unless absolutely necessary and for his mission only. Basically, he cannot help Lina without Zelas' permission or approval.
In Slayers NEXT, Xelloss's purpose was to prevent Lina Inverse from getting killed. Xelloss was 'lent' to Hellmaster Phibrizo by Zelas and currently under his command to keep Lina alive. It seems ridiculous for a Mazoku to go out of his way to keep a silly human alive, isn't it? No. You see, Hellmaster Philbrizo wanted Lina alive because she's the once sorceress he needs to bring destruction to the world. Phibrizo needed Lina to utilize her Giga Slave spell and destroy the world. He knew well enough that the spell called upon the Lord of Nightmares to be summoned into Lina's body. He also knew that Lina could not control the spell, and if once she loses control the entire world will end. Fortunately, this instance is the only time Lina was able to actually control the Giga Slave and summon the Lord of Nightmares. The Lord of Nightmares granted Lina's wish, engulfed Lina and destroyed Hellmaster Phibrizo. With Gourry's determinacy to get Lina back, the Lord of Nightmares released Lina from her captive and returned her to the physical world.^^ Xelloss did not fail in his mission, for he kept Lina alive. After Philbrizo died, Xelloss has no need to ensure Lina's safety and therefore returned to Zelas. 
In Slayers TRY, Xelloss has a new mission. After Phibrizo killed Gaav, there was one loyal servant of Gaav left. Since Gaav was a traitor to the Mazoku, he naturally had to be killed. Any servant would have to be killed as well. However, Valgarv possesses exceptional power. He is the last remaining Ancient Ryuzoku, possessing extreme powers. Valgarv also possesses some powers of Gaav, being his servant. Therefore, he's part Mazoku and part Ryuzoku, a powerful combination indeed. Such a being is useful to the Mazoku, therefore, Xelloss's objective was to find Valgarv and get him to join the Mazoku. Xelloss even offered Lina's life in exchange for Valgarv to join the Mazoku. At first, Lina and company thought that Xelloss was out to kill Valgarv, for he was a servant to the traitor Gaav. But as Filia found out later, Xelloss's objective was slightly different. However, upon Valgarv's refusal to join the Mazoku, Xelloss's mission changed. He is now ordered to kill Valgarv. This is not exactly easy, since Valgarv's power is extremely deadly. Valgarv is out to destroy the world and take revenge on everyone. Valgarv is a huge mess, full of hatred, frustration and anger for those who destroyed his dragon race and those who destroyed Gaav, his master. Valgarv had summoned the DarkStar, a dark lord from another world, to destroy the world Valgarv, Xelloss, Lina and the others resided in. Therefore, Lina and company must destroy DarkStar. Five weapons can be used at the same time to destroy DarkStar, but Lina used one of them (Galveria), combined it with Filia's Ryuzoku power, Xelloss's Mazoku power and her own power to destroy DarkStar. Xelloss did not succeed in his mission in Slayers TRY. Although destroying DarkStar would most likely mean the obliteration of Valgarv as well, he was actually reborn and taken under Filia's care. But, Xelloss prevented DarkStar from wiping out his race, so I suppose it evens out in the end. ^-^<< return