Cosgrach Fionnlagh Culloden, served as the Professor of Potions for Hogwarts School of Witchcraft and Wizardry (2082-88).
Born as a second child to a wealthy, pureblood, Slytherin-since-the-beginning-of-the-world family rooting from Scotland, Cosgrach showed signs of a spoilt child earlier than he showed his magical talents. He had learnt that 'they were wealthy, purebloods, wizards, Slytherins, clearly better than everyone else' and 'didn't like muggles or even worse, muggleborns' when he was only five years old. As he grew older, he had muggle and wizard friends from the neighbourhood, but the wizard side was conscious they were superior to the other side, and wasn't late to show it in their attitude. Needless to say, the tension led to verbal insults, and that led to a massive fight on one hot June day--a fight the neighbourhood had yet to see, and probably never would. The wizard families accused muggles, muggles accused wizards, police fined both. It was never discussed even once, but since then, no muggles stepped onto 'wizard soil' (those families are... different, my child, dear friend. They have owls coming to and leaving their houses. You don't want to mess with them.) and no wizard walked across theirs (muggles are inferior race. We are better than them. We don't need their friendship.) And thus, having private tutours to teach him some basic lessons and playing with his spoilt wizard friends, Cosgrach reached the age of Hogwarts.
He was terrified with the possibility of being sorted into anything other than Slytherin and bringing disgrace to the family, but the Hat knew better than needed less than a second to announce 'Slytherin!' He was very relieved, although disturbed slightly with Daisy's being a Ravenclaw, but the other two boys were in Slytherin too so he didn't mind it too much. He had friends, he was in Slytherin, he had money and had relatively good subjects, so years went by fast. Who cared if they were the bully of the school? Surely their jokes weren't that bad! Everyone deserved a good laugh here and there, and it was those losers problem if they couldn't handle a bit of Slytherin humour. Besides, people shied away from them in the halls better this way, and they were the kings of the school, even though that meant Daisy didn't approve of them anymore either.
Did I say he was happy? Until the fifth year. That blasted, terrible year that changed his life.
He knew his father gambled. But he also knew they had infinite money, so it made sense. What he didn't know, though, was a tiny detail: They had nearly infinite money. So, after all the gambling and losing, his father had lost it all and they desperately tried to hide it from people by selling some of the furniture in muggle shops, but it was no use. As he returned home for Christmas holidays, he found that people--strangers--thieves! were looking at their HOUSE with an intention to buy it. And they did. The signatures under the selling had more meaning than that, though. It meant the end of the world as they knew it. They moved to an apartment in Edinburgh and his mother applied to Ministry too. The first few months were alright, but soon enough, their 'friends' started to turn their backs. 'I really wanted to hang out with you, but I have this to attend to urgently,' was the only answer they started to get. Those friends had apparently owled their children too, because soon enough his mates started making excuses just like their parents, and the other people were still too scared of him to even greet him. He didn't accept defeat without a brave battle and he desperately tried to keep his friends, but it was no use. The worst thing had happened to him: He was both alone and poor, and he didn't even have a shoulder to lie his head down.
He failed some of his OWLs, begged to professors, and was accepted into Potions, Transfiguration, Arithmancy, Herbology and CoMC in his sixth year. He became one of the most silent students in the halls and was usually found reading somewhere. It was a remarkable thing, for he had barely read anything until then. He was also lonely but swallowed it down, and started realising things. That he had wasted his youth on stupid reputation games. That in the darkest moment, even his shadow left him alone. That there was only him that was concerned about him and that it still wasn't late to save himself. He became even more silent--except in the classes of Potions, Herbology and CoMC, read and studied. Hard. He was appointed as the Head Boy in his seventh term, but even this failed to bring a smile upon his face. He just did what he was told to do, and graduated with only signing his name in the seventh year memory book and not writing any other word of wisdom there.
Then he smiled.
He was accepted into University of Witchcraft and Wizardry in United States, in potions major. As he read the letter though, he realised that he should either break into Gringotts or earn some money by working. He chose the latter option wisely, and got into a muggle shop in Edinburgh. He wasn't patient to do things the muggle way, though, and soon enough, his boss was pretty amazed by his working efficiency to raise his salary. He worked there for two years and earned enough money to go to the university, and so he did, for the best four years of his life, because it was a fresh start as no one knew him there. He enjoyed his classes--enjoyed potions!--learnt to respect and like muggleborns, and made good friends. After the four years, he heard about a research group in Turkey on endemic plants, and applied for it. It was a four years of studying both the magical and non-magical plants in Turkey, and he got a spot. During the four years, they stepped on every bit of the rich Anatolia and discovered a few new plants. He was responsible for finding out their properties in potions, and these four years were even better than his time in the US. The pay was low, but he didn't mind it. It wasn't like he wanted infinite money anyway, and as long as his family and 'friends' were away from him, he was content.
Between you and me, he was also content because of Céline, the Canadian member of the group who he would get married to in a romantic wedding in Borneo.
More or less the same research group was offered to study the animals in rainforests in Borneo, and he was probably the most eager member to accept it. They spent another four years on the island--two of those with 'marriage' for him. Although the rain forests had pretty endemic plants too, their focus was on animals and insects this time. He was pretty happy there too, and he got married in the rainy season because he believed rain was 'abundance'--and abundance of too many fights after 2 years and a divorce in Borneo. She accused him of loving potions more than he did, he accused her of suffocating him. A few curses were uttered: he was rescued by a friend from his sticking charm to his potions cauldron 12 hours later, and she was rescued from her disturbing sleep adorned with nightmares by another potion master 13 hours later. They never talked since.
Maybe he really was in love with potions, because what many people called a haste decision was given a week later: He was going to Afghanistan and Kazakhstan to study ancient potions.
And to marry and divorce for the second time in his life. The lady was Natasha--a Ukrainian, but only a Veritaserum can make him talk about those six months.
The four years between Kazakhstan and his career in Hogwarts is unclear. Some say he got into a muggle university to study chemistry, some say he taught chemistry in a high school, many say he locked himself up in his house in Kazakhstan and experimented with potions, and a few are bold enough to suggest he's learnt some muggle medicine in United States with Daisy (who happened to be there then, who still is). He never certified or declined the rumours. Daisy only cursed after hearing his name every time. What is known for sure is, he heard about Lafay's quitting and applied for the spot.
And you've just read the new potion master's background.