Kefka is the most beautifully realised, nuanced, off-kilter, and flat-out terrifying villain in the entire history of Final Fantasy. He might not be a genetically modified, single-minded supersoldier or an omnipotent sorcerer capable of bending time and space to his will, but that’s exactly why he’s so scary. Kefka is a big bundle of psychosis and very human frailties who clawed together every last scrap of (ultimately apocalyptic) power he gained through his own desperate rage.
Long before that point though, he was still the most effortlessly cruel of all Final Fantasy bad guys, poisoning a besieged town’s water supply even after the battle was all-but won. Following that, he brought about the end of the world as a casual side-effect of his ultimate grasp for power, then spent the days after the end of days nihilistically torturing and destroying the world’s put-upon survivors simply for his own amusement. A horrific, anarchic nightmare of a man/god, Kefka is regardless far from unrelatable. Essentially an angry, petulant toddler in the body of an apocalyptic harbinger of destruction, underneath his countless atrocities there always lay a scared, sad, out-of-control kid just trying to make sense of his lot.
I agree with you that he's not incredibly relatable in that he's bat-sh*t crazy, but other than that I felt like they nailed him. I think what they meant by relatable was that he acts like an 8-year old boy with a magnifying glass over an ant hill most of the game.
He's definitely cruel, off-kilter, and power hungry. He's apocalyptic and maniacal, and definitely does half the stuff in the game for his own amusement... I will never forget that laugh. Most of his power he gained himself through betrayal and crazy amounts of rage. I dunno, maybe the description was a bit wordy, but all-in-all I thought what they described is what I think of when I think of Kefka.