Bart is a rock troll Geralt encounters during the quest, Count Reuven's Treasure, and he is one of my absolute favorite characters from The Witcher 3. To be fair, I'm a huge fan of all the trolls Geralt comes across throughout his adventures. I love that he can conciliate them rather than view them as a typical monster threat.
Part of what makes trolls so great is their toddler-like communication skills and understanding of the world. Think Trollololo near Oxenfurt, who guards the Redanian soldiers' stolen boats by making fences out of some of them. Flawed logic, true. But ingenious by troll standards. Their childlike outlook also explains why Geralt would try to reason with them rather than writing them off as dangerous monsters.
Rock trolls' dimwitted ways and generally innocent intentions are a wonderful juxtaposition to their brutish appearance and strength. Even Trollololo, who some view as a man-eater, was only trying to pacify the squabble between the soldiers and the peasants. Despite his inclination to not waste good meat, the Redanian-pledged troll displayed his loyalty by continuing to guard the boats.
Bart also shows great loyalty and even a sense of gratitude towards Sigi Reuven - Sigismund Dijkstra's alias - for 'saving' him from the extreme Zerrikanian climate. The troll's remorseful displays also set Bart apart. When Geralt first meets Bart, the troll is bashing his own head against a stone wall as a sort of self-punishment for failing his guard duty of Sigi's treasure. Bart's self-recrimination reminded me a lot of Dobby, the widely beloved house elf from Harry Potter. Despite a vast size difference and incomparable grammar usage, these two characters share strong virtues and unshakeable loyalty to those they owe fealty.
Bart's also the only troll Geralt encounters where there isn't the ultimatum of appeasing or killing, as he's already an ally. He's a self-professed 'nice' troll and a thinker as well, albeit the simple contemplation of rocks. However, guarding treasure in a dungeon all alone gave him a lot of time to mull over these troll-confounding minerals. I find it highly endearing Bart could be considered a scholar among trolls for delving into the mysteries which most closely concern these simple-minded cave dwellers.
What it really boils down to, for me, is how immensely enjoyable the dialogue between Geralt and the trolls is when he attempts to conciliate them. The exchanges with trolls are an extremely humorous reprieve amongst grueling witcher work, Bart's optional dialogue in particular. Furthermore, it's simply endearing to watch the usually gruff and terse witcher speak so mildly, despite how his hot-headed witcher pal, Lambert might feel about his troll negotiation skills.