Heya there! So happy you're curious about their names ~ Time to drop some lore (and who doesn't love that for their characters? (≧◡≦) ♡)
The name "Merka" has a Turkic origin with some adjustments— It comes from the word "Merak," which means curiosity (something that pops up in his future route and was even hinted in the demo as to how he knew certain things about Sev) and desire (a little nod to his situationship with Rosario). It's also an anagram for 'Maker' (maybe related to his route... no spoilers though!)
As for "Yulen," he's actually an OC from a scrapped story idea (ah, the Wattpad era mafia stories XD). With a bit of tweaking, 'Yul' comes from 'Yulian,' which means youthful—hinting that he may or may not be the first one born in the painting (Adonis always loves to argue about that (¬‿¬). It also nods to the Turkic word "Yalan," which means lie, since Yulen insists he always tells the truth and nothing but the truth (or does he...?) I originally wanted to combine the two as 'Yul-lan,' but kept hearing 'Mulan' in my head for some reason, so I had to adjust it LOL
Bonus fun fact! I was actually considering naming him "Yalan," but I realized people might pronounce it as 'YAY-len' instead of 'YUH-lun,' so I scrapped that idea real quick XDD