The personality that Xelloss shows to Lina and Company is that of a lighthearted and mischievous one. This is the side of Xelloss that we the viewers get to know best during Slayers NEXT. During Slayers TRY, however, Xelloss' attitude takes on a darker twist as he gets new orders from his Lord. Xelloss' personality is, in no way, easy to explain. There are many different sides to him but we must remember that Xelloss is a mazoku and he is therefore evil, no matter what sort of act he may put on otherwise.
Mystery clouds Xelloss' mind and purpose. He comes and goes as he pleases and does whatever he wants to do. He often replies to questions with "Sore wa himitsu desu!" ("That is a secret!"), much to the annoyance of others, and never tells anything about himself. Not even Lina herself knows what Xelloss is actually up to.
In tribute to his chaotic and mischievous nature, Xelloss sometimes refers to himself as the Trickster Priest. When it comes to tricks and mischief, Xelloss is a Jack-of-all-Trades. Now and then, Xelloss will pull simple pranks on unsuspecting civilians for a quick dose of negative energy (in this case, panic and anger). However, Xelloss' tricks are not normally joke shop material. He often manipulates people, but he never lies. Any information he gives has the most important parts left out: he may tell you that a chest of gold is in a cave close by, but he'll neglect to tell you that it's guarded by a large, fire-breathing dragon. Xelloss likes to play with people's minds to get what he needs.
With that in mind, Xelloss is.. a bit odd. Whether or not his strange sense of humor is part of his Trickster guise is anyone's guess. He often appears to be a cheerful individual, perhaps too cheerful, but effective in annoying others. He often acts innocent but people such as Zelgadiss and Lina know that Xelloss really isn't what he claims to be. His whole cheerful personality is just a mask... his primary goal in acting this way is to stir up trouble and get some negative energy flowing from the humans (or ryuzoku) around him.
Xelloss often appears to be strangely fearless and confident. When posing as a human, he seemed to have a great confidence when it came to cooking a meal to pass a test in one episode of Slayers NEXT. At seeing his display of confidence, others became confident too -- That is, until they found out that Xelloss really couldn't cook at all. However, Xelloss' great confidence and lack of fear stems from him being a mazoku.
Mazoku are creatures that have no fear. They are confident beyond all reason. To have fear and a lack of self-confidence would be human and since mazoku look down upon humans as rats, well... to have fear and no confidence just wouldn't suffice. However, with confidence and fearlessness comes loyalty and power. Loyal is Xelloss to his Lord, Juu-oh (Greater Beast Zelas Metallium). He would never stray from her side nor disobey her orders. Defying Juu-oh is something that Xelloss would never do, no matter how devious he may be.
In essence, Xelloss may be cheerful and playful on the outside but he hides a nasty streak within him. He will not hesitate to sacrifice the lives of his traveling companions if it means his Lord would benefit. He thrives off of negative emotions (fear, pain, anger, sorrow, etc) and that is his reason for staying around humans. No matter how amicable he may be, Xelloss will always be a dark individual.