I think for this you have to go back to the actual Legend of Faust where the character is derived from.
In the legend, a scholar named Faust makes a deal with the devil for ultimate power essentially. In the deal, he is given a demon names Mephistopheles whom will act as his servant until his bargain come up and at which point Mephistopheles will take his soul and damn him.
In this case I think that if Katou want's to play with it in a sense, Faust could have originally been a high ranking exorcist whom controlled Mephisto. Possible that Satan killed Faust and Mephisto, who had grown fond of him, wants to take him down?
It's kind of hard to say at this point, but I think that would be an interesting basis.
Honestly I say his character is more on the chaotic neutral aspect I mean he's helping rin out ,but at the same time seems like he's rebelling Satan well I need to reread the series again ,but that's my impression of him thus far.
First of all, Mephisto = Izaya = GGGGOOOOOODDDDD!!!
Secondly, it's most likely a right the for the wrong reason sort of situation. Slightly less likely, a little over two hundred years ago Mephisto experienced a great trauma which caused him to renounce Gehenna in disgust or at the very least with boredom.
Hm.... there are theories to support Mephisto's goals on all sides, whoever he may be allied with, but i actually really hope that he is and remains a good guy... IDK, he's just too compasionate to be a traitor/double agent - especially if he has been at it for 200 years...
Well to be frank , all his acts seem to represent him having secret alternate motives.
Now we have been told that there are many ppl in the exorcist line up who have demon blood . Our protagonist is special because of the blood for him belonging to Satan . Lets just assume that Pheles is maybe just from a higher ranked demon and hence has more characteristics like one .
Another theory can say he has a relation to angels / fallen angels as he addressed Rin as brother and not prince .
Maybe he was created by so and so angel to assist in killing of Satan for maybe Heaven's pass or whatever
Wanted to add the fact that Gehena is the Hindi world for Jewel . Maybe we might see the relation as well later
He was Shirou's friend, the Order trusts him and so do I. Baldy doesn't trust him and I don't trust Baldy, so that's another argument for Mephisto being on the good side. Apart from that, the way he protects Rin, cares for Yukio's state of mind, cares for his school and pupils shows that he's not evil. I think he wants to take satan's place and change things in his homeland.
I have a theory that 200 years earlier something happened that made him switch sides. Maybe he loved a human and satan killed him/her?
As the old saying goes "can you really predict a clown's ulterior motives?" XD. In all seriousness though he really is one of the more intriguing characters throughout the story so far. At moments he seems as if he genuinely cares for the order and is on the exorcist's side. Then at other times he does something which arouses the readers suspicion. For example Amaimon seems to be pure evil yet he obeys Mephisto which is something I can't see him doing if Mephisto was not loyal to satan. Of course there is always the possibility of an ulterior motive which leads me back to the beginning of this comment.
Wow didn't know bout the Legend of Faust I'd be inclined to think it's something along those lines for sure! But then maybe there will be more of a sinister element to it. I don't think mephisto is really a good guy at all I think he does have some goals of his own and while he still is on satan's side he's more interested in what happens on the weaker side and how the control of power will change with Rin. Maybe he was only even friends with shiro for Rin's purpose.
He seems to be a good gy with an ulterior motive but i dont think he is on stans side :/ nor on Rins. He may want satan eliminated so he can grow to power maybe?
I dont know, for all we know he may even want to become either fully human (because all n all hes a good guy troubled by his past)or fully demon (he wants to rule the world and be the most epic as well as elegantly dressed Bad Guy ever shown.
For now its too soon to judge and whats for sure is that he himself has a plan of his own.
Only watching the anime, I'd say he is very suspicious. As he clearly said on the first episode about "not mixing public issues and private matters". I translated this that he obviously has more than one reason to take care of Rin after Fujimoto died. As part of the Order, he actually has to kill the son of Satan, but since Rin insisted on becoming an exorcist, maybe he thought he may as well teach Rin well so Rin can be useful to the order. I think that was why he had a teacher attack Rin to "analyze his power", although I doubt that was his real intention. That's for the "public issues". For "private matter" is the part in which he seems to be someone from Gehenna, having brother who looks like a demon, and calling Rin brother as well. That being said, I look forward to see who this person actually is.
He's too much of an enigma to be a truly good guy.
It's as if he doesn't belong to the good side or the bad side but to his own side.
But I have the inkling that he wants to kill Satan as well. At first, he was going to kill Rin but then after he saw Rin's motivation to be an exorcist, he decided to train him. Then, he went so far as to test Rin's strength. Maybe he thinks he can use Rin in one way or another?
I'm a bit confused with his relationship with Amaimon though. Are they "partners-in-crime"? They seem to have a lot of shady communication.
He and Amaimon seem to be brothers, Amaimon also refers to himself as a son of Satan and Rin as a "brother of sorts". I suspect that Mephisto is another of the "eight kings". I have to agree, I think Mephisto intends to kill Satan. He probably thinks Rin is potentially strong enough to fight Satan, and is developing his true power. Neigauz described himself as "the accelerator", forcing Rin to bring out his powers. Amaimon was used in a similar manner. More than testing him, I think Mephisto is trying to draw as much of Rin's power out as possible - to turn him into his ultimate weapon.
I think the most probable intention of him is indeed to kill Satan. Another reason than the reasons already said above: if he's not going to kill Satan, why Neigauz is so loyal to him? We all know so well that Neigauz has a deep hatred toward Satan.
Yep, that's the word. "Ultimate weapon". Seems like Mephisto and Satan are going to compete to get Rin.
But Amaimon is more of an enigma than Mephisto. He doesn't seem loyal to Mephisto.