A pretty nondescript man in his mid 30s. Normally wears a long leather apron to protect himself from splash damage when his experiments (inevitably) fail.
When Kostovic got kicked out of the College of Magic for incompetence, he was faced with the problem of making a living (by no means an easy task for a failed wizard). Just being bad at magic wasn’t really the problem (inept wizards are common enough), no the problem was that he was completely useless - he could barely master the simplest of cantrips (and even they tended to go wrong in his hands).
Kostovic would have been destined to become just another failed wizard had he not then been apprenticed to Corbin the Alchemist. As it happens, Kostovic was only slightly better at this than he was at magic (being merely inept) and Corbin soon tired of cleaning up after the explosions. And so Kostovic was once again looking for a way to make his fortune.
Ever the optimist, Kostovic decided that the reason he kept on failing was that his bosses kept getting in the way. The solution, of course, was simple - he would set up on his own.
Initially (due mainly to limited funds) he eked out a modest living producing and selling a variety of non-magical substances (smelling salts, cleaning fluids, preserving agents, cheap perfumes, etc).
But Kostovic was convinced that the raw spark of magic lay dormant in his soul and he was determined to find it. To this end he began to experiment with magical potions. Since he is unable to cast magic (and therefore unable to create even the simplest elixirs himself) this seemed doomed to failure but for one small ray of hope.
The alchemists had long known how to replicate existing potions, without all the spells and incantations normally required. This is virtually identical to the original creation process (requiring all the proper ingredients prepared in the correct manner) but uses a "seeding dose" to replace the spells. The yield is not particularly high (say 1d4 doses if successful, burnt eyebrows if not), but it is sufficient to feed a steady market for the more common potions (i.e. those that produce an easily marketable effect, do not require exotic or arcane ingredients, and are reasonable safe the make).
Having been apprenticed to an alchemist, Kostovic of course knows how to do this (even if he’s not particularly good at it), and this became the starting point for his experiments. What he tried to do was modify this process using cantrips to boost the yield and, much to everyone’s amazement, he actually succeeded. The results are a little better, not much, just a little (say 2d3 doses if successful) - or it would be but for Kostovic’s ineptitude.
The truly notable breakthrough (if it can be called that that) came as a result of a laboratory mistake (perhaps the only thing Kostovic truly excels in). Whilst attempting to replicate a standard magical potion (any type, whatever fits your campaign) he mispronounced the wording of the cantrip. However instead of blowing up in his face (the normal result of such a mistake), a new and unique magical potion was formed - Kostovic’s Elixir of Magic.
In the long and noble tradition of mad scientists throughout the multi-verse, Kostovic immediately tried the new potion himself just to see what it would do. Of course it didn’t take him long to learn its’ magical properties, but its’ addictive properties did not become apparent until a few days later. By then, of course, it was far too late - he had become the first addict of his own designer drug.