First off, some writers present their characters as having new, original philosophies. For which my advice is,
Don't do it.
No, you have not devised a new, original philosophy. People have thought philosophically for thousands and thousands of years. You have not come up with a startling insight that all of them have managed to miss. And if your character really is sagacious, he will not care. He would no more desire to have a new philosophy than a new multiplication table. He's after bigger game: a true philosophy. And he would not care in the least if everyone on earth believed it before him.
If, on the other hand, you want to rip off existing philosophy, you still have some interesting work to do. For one thing, people have thought philosophically for thousands and thousands of years. True, a tiny fraction of it has survived, but still more than makes going through easy. And a lot more of it than appears at first glance assumes that you've read other stuff, or is in response to it, or otherwise really needs you to have read a lot.
On the other hand, the primary sources are much better than a summary secondary source on the history of philosophy. Leaving aside that the second source will filter everything through the writer's own views, you get a better feel for style. I can not believe in a sage who sounds exactly like any old clod. (If for nothing else, he would consider words important and worthy of reflection. "To express oneself badly is not only faulty as far as the language goes, but does some harm to the soul.")
Also, the primary source may help you avoid blunting the edges of the philosophy. Avoid picking and choosing what is pretty and easy to cope with -- the picking and choosing is a very good way to expose the depths of your shallowness. Various urban fantasies I have read have cited Jesus in a manner which carefully leaves out such things as his recommending amputation as a means to deal with temptation, and that when considering the question "What would Jesus do?" one of the options is, after all, "throw a fit and start to toss around the furniture." And I have run across the pleasant and platitudinous in epic fantasies where the heroes are out to save the world, and it ought to be obvious to them that being pleasant and nice would be the destruction of all that is pleasant and nice -- but they don't realize it because the "sage" is saying it.
On the whole, it is generally wise to have your sage shut up. Often. Lectures from sages not only have all the usual lack of charm of long lectures; they have a dreadful tendency to expose what their sagacity is. And as Socrates observed long ago, the wisest man of all is the man who knows his own limits.